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18be03ef23
The previous commits do exactly what this entry in the TODO file asks for, thus we can remove it now as it is no longer applicable. Signed-off-by: Fernando Ramos <greenfoo@u92.eu> Reviewed-by: Sean Paul <sean@poorly.run> Signed-off-by: Sean Paul <seanpaul@chromium.org> Link: https://patchwork.freedesktop.org/patch/msgid/20210924064324.229457-18-greenfoo@u92.eu
394 lines
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ReStructuredText
394 lines
15 KiB
ReStructuredText
======================================
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Wound/Wait Deadlock-Proof Mutex Design
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======================================
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Please read mutex-design.rst first, as it applies to wait/wound mutexes too.
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Motivation for WW-Mutexes
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-------------------------
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GPU's do operations that commonly involve many buffers. Those buffers
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can be shared across contexts/processes, exist in different memory
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domains (for example VRAM vs system memory), and so on. And with
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PRIME / dmabuf, they can even be shared across devices. So there are
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a handful of situations where the driver needs to wait for buffers to
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become ready. If you think about this in terms of waiting on a buffer
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mutex for it to become available, this presents a problem because
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there is no way to guarantee that buffers appear in a execbuf/batch in
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the same order in all contexts. That is directly under control of
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userspace, and a result of the sequence of GL calls that an application
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makes. Which results in the potential for deadlock. The problem gets
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more complex when you consider that the kernel may need to migrate the
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buffer(s) into VRAM before the GPU operates on the buffer(s), which
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may in turn require evicting some other buffers (and you don't want to
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evict other buffers which are already queued up to the GPU), but for a
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simplified understanding of the problem you can ignore this.
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The algorithm that the TTM graphics subsystem came up with for dealing with
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this problem is quite simple. For each group of buffers (execbuf) that need
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to be locked, the caller would be assigned a unique reservation id/ticket,
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from a global counter. In case of deadlock while locking all the buffers
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associated with a execbuf, the one with the lowest reservation ticket (i.e.
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the oldest task) wins, and the one with the higher reservation id (i.e. the
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younger task) unlocks all of the buffers that it has already locked, and then
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tries again.
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In the RDBMS literature, a reservation ticket is associated with a transaction.
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and the deadlock handling approach is called Wait-Die. The name is based on
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the actions of a locking thread when it encounters an already locked mutex.
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If the transaction holding the lock is younger, the locking transaction waits.
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If the transaction holding the lock is older, the locking transaction backs off
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and dies. Hence Wait-Die.
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There is also another algorithm called Wound-Wait:
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If the transaction holding the lock is younger, the locking transaction
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wounds the transaction holding the lock, requesting it to die.
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If the transaction holding the lock is older, it waits for the other
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transaction. Hence Wound-Wait.
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The two algorithms are both fair in that a transaction will eventually succeed.
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However, the Wound-Wait algorithm is typically stated to generate fewer backoffs
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compared to Wait-Die, but is, on the other hand, associated with more work than
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Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
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algorithm in that transactions are wounded by other transactions, and that
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requires a reliable way to pick up the wounded condition and preempt the
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running transaction. Note that this is not the same as process preemption. A
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Wound-Wait transaction is considered preempted when it dies (returning
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-EDEADLK) following a wound.
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Concepts
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--------
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Compared to normal mutexes two additional concepts/objects show up in the lock
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interface for w/w mutexes:
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Acquire context: To ensure eventual forward progress it is important that a task
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trying to acquire locks doesn't grab a new reservation id, but keeps the one it
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acquired when starting the lock acquisition. This ticket is stored in the
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acquire context. Furthermore the acquire context keeps track of debugging state
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to catch w/w mutex interface abuse. An acquire context is representing a
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transaction.
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W/w class: In contrast to normal mutexes the lock class needs to be explicit for
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w/w mutexes, since it is required to initialize the acquire context. The lock
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class also specifies what algorithm to use, Wound-Wait or Wait-Die.
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Furthermore there are three different class of w/w lock acquire functions:
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* Normal lock acquisition with a context, using ww_mutex_lock.
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* Slowpath lock acquisition on the contending lock, used by the task that just
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killed its transaction after having dropped all already acquired locks.
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These functions have the _slow postfix.
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From a simple semantics point-of-view the _slow functions are not strictly
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required, since simply calling the normal ww_mutex_lock functions on the
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contending lock (after having dropped all other already acquired locks) will
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work correctly. After all if no other ww mutex has been acquired yet there's
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no deadlock potential and hence the ww_mutex_lock call will block and not
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prematurely return -EDEADLK. The advantage of the _slow functions is in
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interface safety:
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- ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow
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has a void return type. Note that since ww mutex code needs loops/retries
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anyway the __must_check doesn't result in spurious warnings, even though the
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very first lock operation can never fail.
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- When full debugging is enabled ww_mutex_lock_slow checks that all acquired
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ww mutex have been released (preventing deadlocks) and makes sure that we
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block on the contending lock (preventing spinning through the -EDEADLK
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slowpath until the contended lock can be acquired).
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* Functions to only acquire a single w/w mutex, which results in the exact same
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semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL
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context.
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Again this is not strictly required. But often you only want to acquire a
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single lock in which case it's pointless to set up an acquire context (and so
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better to avoid grabbing a deadlock avoidance ticket).
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Of course, all the usual variants for handling wake-ups due to signals are also
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provided.
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Usage
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-----
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The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
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DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die)
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As a rough rule of thumb, use Wound-Wait iff you
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expect the number of simultaneous competing transactions to be typically small,
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and you want to reduce the number of rollbacks.
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Three different ways to acquire locks within the same w/w class. Common
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definitions for methods #1 and #2::
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static DEFINE_WW_CLASS(ww_class);
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struct obj {
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struct ww_mutex lock;
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/* obj data */
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};
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struct obj_entry {
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struct list_head head;
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struct obj *obj;
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};
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Method 1, using a list in execbuf->buffers that's not allowed to be reordered.
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This is useful if a list of required objects is already tracked somewhere.
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Furthermore the lock helper can use propagate the -EALREADY return code back to
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the caller as a signal that an object is twice on the list. This is useful if
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the list is constructed from userspace input and the ABI requires userspace to
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not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl)::
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int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
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{
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struct obj *res_obj = NULL;
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struct obj_entry *contended_entry = NULL;
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struct obj_entry *entry;
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ww_acquire_init(ctx, &ww_class);
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retry:
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list_for_each_entry (entry, list, head) {
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if (entry->obj == res_obj) {
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res_obj = NULL;
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continue;
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}
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ret = ww_mutex_lock(&entry->obj->lock, ctx);
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if (ret < 0) {
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contended_entry = entry;
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goto err;
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}
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}
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ww_acquire_done(ctx);
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return 0;
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err:
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list_for_each_entry_continue_reverse (entry, list, head)
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ww_mutex_unlock(&entry->obj->lock);
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if (res_obj)
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ww_mutex_unlock(&res_obj->lock);
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if (ret == -EDEADLK) {
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/* we lost out in a seqno race, lock and retry.. */
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ww_mutex_lock_slow(&contended_entry->obj->lock, ctx);
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res_obj = contended_entry->obj;
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goto retry;
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}
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ww_acquire_fini(ctx);
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return ret;
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}
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Method 2, using a list in execbuf->buffers that can be reordered. Same semantics
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of duplicate entry detection using -EALREADY as method 1 above. But the
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list-reordering allows for a bit more idiomatic code::
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int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
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{
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struct obj_entry *entry, *entry2;
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ww_acquire_init(ctx, &ww_class);
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list_for_each_entry (entry, list, head) {
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ret = ww_mutex_lock(&entry->obj->lock, ctx);
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if (ret < 0) {
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entry2 = entry;
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list_for_each_entry_continue_reverse (entry2, list, head)
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ww_mutex_unlock(&entry2->obj->lock);
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if (ret != -EDEADLK) {
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ww_acquire_fini(ctx);
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return ret;
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}
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/* we lost out in a seqno race, lock and retry.. */
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ww_mutex_lock_slow(&entry->obj->lock, ctx);
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/*
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* Move buf to head of the list, this will point
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* buf->next to the first unlocked entry,
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* restarting the for loop.
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*/
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list_del(&entry->head);
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list_add(&entry->head, list);
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}
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}
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ww_acquire_done(ctx);
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return 0;
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}
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Unlocking works the same way for both methods #1 and #2::
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void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
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{
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struct obj_entry *entry;
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list_for_each_entry (entry, list, head)
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ww_mutex_unlock(&entry->obj->lock);
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ww_acquire_fini(ctx);
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}
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Method 3 is useful if the list of objects is constructed ad-hoc and not upfront,
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e.g. when adjusting edges in a graph where each node has its own ww_mutex lock,
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and edges can only be changed when holding the locks of all involved nodes. w/w
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mutexes are a natural fit for such a case for two reasons:
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- They can handle lock-acquisition in any order which allows us to start walking
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a graph from a starting point and then iteratively discovering new edges and
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locking down the nodes those edges connect to.
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- Due to the -EALREADY return code signalling that a given objects is already
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held there's no need for additional book-keeping to break cycles in the graph
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or keep track off which looks are already held (when using more than one node
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as a starting point).
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Note that this approach differs in two important ways from the above methods:
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- Since the list of objects is dynamically constructed (and might very well be
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different when retrying due to hitting the -EDEADLK die condition) there's
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no need to keep any object on a persistent list when it's not locked. We can
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therefore move the list_head into the object itself.
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- On the other hand the dynamic object list construction also means that the -EALREADY return
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code can't be propagated.
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Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a
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list of starting nodes (passed in from userspace) using one of the above
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methods. And then lock any additional objects affected by the operations using
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method #3 below. The backoff/retry procedure will be a bit more involved, since
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when the dynamic locking step hits -EDEADLK we also need to unlock all the
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objects acquired with the fixed list. But the w/w mutex debug checks will catch
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any interface misuse for these cases.
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Also, method 3 can't fail the lock acquisition step since it doesn't return
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-EALREADY. Of course this would be different when using the _interruptible
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variants, but that's outside of the scope of these examples here::
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struct obj {
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struct ww_mutex ww_mutex;
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struct list_head locked_list;
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};
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static DEFINE_WW_CLASS(ww_class);
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void __unlock_objs(struct list_head *list)
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{
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struct obj *entry, *temp;
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list_for_each_entry_safe (entry, temp, list, locked_list) {
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/* need to do that before unlocking, since only the current lock holder is
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allowed to use object */
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list_del(&entry->locked_list);
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ww_mutex_unlock(entry->ww_mutex)
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}
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}
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void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
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{
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struct obj *obj;
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ww_acquire_init(ctx, &ww_class);
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retry:
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/* re-init loop start state */
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loop {
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/* magic code which walks over a graph and decides which objects
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* to lock */
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ret = ww_mutex_lock(obj->ww_mutex, ctx);
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if (ret == -EALREADY) {
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/* we have that one already, get to the next object */
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continue;
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}
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if (ret == -EDEADLK) {
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__unlock_objs(list);
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ww_mutex_lock_slow(obj, ctx);
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list_add(&entry->locked_list, list);
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goto retry;
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}
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/* locked a new object, add it to the list */
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list_add_tail(&entry->locked_list, list);
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}
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ww_acquire_done(ctx);
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return 0;
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}
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void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
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{
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__unlock_objs(list);
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ww_acquire_fini(ctx);
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}
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Method 4: Only lock one single objects. In that case deadlock detection and
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prevention is obviously overkill, since with grabbing just one lock you can't
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produce a deadlock within just one class. To simplify this case the w/w mutex
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api can be used with a NULL context.
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Implementation Details
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----------------------
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Design:
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^^^^^^^
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ww_mutex currently encapsulates a struct mutex, this means no extra overhead for
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normal mutex locks, which are far more common. As such there is only a small
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increase in code size if wait/wound mutexes are not used.
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We maintain the following invariants for the wait list:
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(1) Waiters with an acquire context are sorted by stamp order; waiters
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without an acquire context are interspersed in FIFO order.
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(2) For Wait-Die, among waiters with contexts, only the first one can have
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other locks acquired already (ctx->acquired > 0). Note that this waiter
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may come after other waiters without contexts in the list.
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The Wound-Wait preemption is implemented with a lazy-preemption scheme:
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The wounded status of the transaction is checked only when there is
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contention for a new lock and hence a true chance of deadlock. In that
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situation, if the transaction is wounded, it backs off, clears the
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wounded status and retries. A great benefit of implementing preemption in
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this way is that the wounded transaction can identify a contending lock to
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wait for before restarting the transaction. Just blindly restarting the
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transaction would likely make the transaction end up in a situation where
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it would have to back off again.
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In general, not much contention is expected. The locks are typically used to
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serialize access to resources for devices, and optimization focus should
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therefore be directed towards the uncontended cases.
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Lockdep:
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^^^^^^^^
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Special care has been taken to warn for as many cases of api abuse
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as possible. Some common api abuses will be caught with
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CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended.
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Some of the errors which will be warned about:
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- Forgetting to call ww_acquire_fini or ww_acquire_init.
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- Attempting to lock more mutexes after ww_acquire_done.
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- Attempting to lock the wrong mutex after -EDEADLK and
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unlocking all mutexes.
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- Attempting to lock the right mutex after -EDEADLK,
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before unlocking all mutexes.
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- Calling ww_mutex_lock_slow before -EDEADLK was returned.
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- Unlocking mutexes with the wrong unlock function.
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- Calling one of the ww_acquire_* twice on the same context.
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- Using a different ww_class for the mutex than for the ww_acquire_ctx.
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- Normal lockdep errors that can result in deadlocks.
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Some of the lockdep errors that can result in deadlocks:
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- Calling ww_acquire_init to initialize a second ww_acquire_ctx before
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having called ww_acquire_fini on the first.
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- 'normal' deadlocks that can occur.
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FIXME:
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Update this section once we have the TASK_DEADLOCK task state flag magic
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implemented.
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