linux/kernel/locking/rwsem-xadd.c
Greg Kroah-Hartman b24413180f License cleanup: add SPDX GPL-2.0 license identifier to files with no license
Many source files in the tree are missing licensing information, which
makes it harder for compliance tools to determine the correct license.

By default all files without license information are under the default
license of the kernel, which is GPL version 2.

Update the files which contain no license information with the 'GPL-2.0'
SPDX license identifier.  The SPDX identifier is a legally binding
shorthand, which can be used instead of the full boiler plate text.

This patch is based on work done by Thomas Gleixner and Kate Stewart and
Philippe Ombredanne.

How this work was done:

Patches were generated and checked against linux-4.14-rc6 for a subset of
the use cases:
 - file had no licensing information it it.
 - file was a */uapi/* one with no licensing information in it,
 - file was a */uapi/* one with existing licensing information,

Further patches will be generated in subsequent months to fix up cases
where non-standard license headers were used, and references to license
had to be inferred by heuristics based on keywords.

The analysis to determine which SPDX License Identifier to be applied to
a file was done in a spreadsheet of side by side results from of the
output of two independent scanners (ScanCode & Windriver) producing SPDX
tag:value files created by Philippe Ombredanne.  Philippe prepared the
base worksheet, and did an initial spot review of a few 1000 files.

The 4.13 kernel was the starting point of the analysis with 60,537 files
assessed.  Kate Stewart did a file by file comparison of the scanner
results in the spreadsheet to determine which SPDX license identifier(s)
to be applied to the file. She confirmed any determination that was not
immediately clear with lawyers working with the Linux Foundation.

Criteria used to select files for SPDX license identifier tagging was:
 - Files considered eligible had to be source code files.
 - Make and config files were included as candidates if they contained >5
   lines of source
 - File already had some variant of a license header in it (even if <5
   lines).

All documentation files were explicitly excluded.

The following heuristics were used to determine which SPDX license
identifiers to apply.

 - when both scanners couldn't find any license traces, file was
   considered to have no license information in it, and the top level
   COPYING file license applied.

   For non */uapi/* files that summary was:

   SPDX license identifier                            # files
   ---------------------------------------------------|-------
   GPL-2.0                                              11139

   and resulted in the first patch in this series.

   If that file was a */uapi/* path one, it was "GPL-2.0 WITH
   Linux-syscall-note" otherwise it was "GPL-2.0".  Results of that was:

   SPDX license identifier                            # files
   ---------------------------------------------------|-------
   GPL-2.0 WITH Linux-syscall-note                        930

   and resulted in the second patch in this series.

 - if a file had some form of licensing information in it, and was one
   of the */uapi/* ones, it was denoted with the Linux-syscall-note if
   any GPL family license was found in the file or had no licensing in
   it (per prior point).  Results summary:

   SPDX license identifier                            # files
   ---------------------------------------------------|------
   GPL-2.0 WITH Linux-syscall-note                       270
   GPL-2.0+ WITH Linux-syscall-note                      169
   ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause)    21
   ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)    17
   LGPL-2.1+ WITH Linux-syscall-note                      15
   GPL-1.0+ WITH Linux-syscall-note                       14
   ((GPL-2.0+ WITH Linux-syscall-note) OR BSD-3-Clause)    5
   LGPL-2.0+ WITH Linux-syscall-note                       4
   LGPL-2.1 WITH Linux-syscall-note                        3
   ((GPL-2.0 WITH Linux-syscall-note) OR MIT)              3
   ((GPL-2.0 WITH Linux-syscall-note) AND MIT)             1

   and that resulted in the third patch in this series.

 - when the two scanners agreed on the detected license(s), that became
   the concluded license(s).

 - when there was disagreement between the two scanners (one detected a
   license but the other didn't, or they both detected different
   licenses) a manual inspection of the file occurred.

 - In most cases a manual inspection of the information in the file
   resulted in a clear resolution of the license that should apply (and
   which scanner probably needed to revisit its heuristics).

 - When it was not immediately clear, the license identifier was
   confirmed with lawyers working with the Linux Foundation.

 - If there was any question as to the appropriate license identifier,
   the file was flagged for further research and to be revisited later
   in time.

In total, over 70 hours of logged manual review was done on the
spreadsheet to determine the SPDX license identifiers to apply to the
source files by Kate, Philippe, Thomas and, in some cases, confirmation
by lawyers working with the Linux Foundation.

Kate also obtained a third independent scan of the 4.13 code base from
FOSSology, and compared selected files where the other two scanners
disagreed against that SPDX file, to see if there was new insights.  The
Windriver scanner is based on an older version of FOSSology in part, so
they are related.

Thomas did random spot checks in about 500 files from the spreadsheets
for the uapi headers and agreed with SPDX license identifier in the
files he inspected. For the non-uapi files Thomas did random spot checks
in about 15000 files.

In initial set of patches against 4.14-rc6, 3 files were found to have
copy/paste license identifier errors, and have been fixed to reflect the
correct identifier.

Additionally Philippe spent 10 hours this week doing a detailed manual
inspection and review of the 12,461 patched files from the initial patch
version early this week with:
 - a full scancode scan run, collecting the matched texts, detected
   license ids and scores
 - reviewing anything where there was a license detected (about 500+
   files) to ensure that the applied SPDX license was correct
 - reviewing anything where there was no detection but the patch license
   was not GPL-2.0 WITH Linux-syscall-note to ensure that the applied
   SPDX license was correct

This produced a worksheet with 20 files needing minor correction.  This
worksheet was then exported into 3 different .csv files for the
different types of files to be modified.

These .csv files were then reviewed by Greg.  Thomas wrote a script to
parse the csv files and add the proper SPDX tag to the file, in the
format that the file expected.  This script was further refined by Greg
based on the output to detect more types of files automatically and to
distinguish between header and source .c files (which need different
comment types.)  Finally Greg ran the script using the .csv files to
generate the patches.

Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2017-11-02 11:10:55 +01:00

708 lines
20 KiB
C

// SPDX-License-Identifier: GPL-2.0
/* rwsem.c: R/W semaphores: contention handling functions
*
* Written by David Howells (dhowells@redhat.com).
* Derived from arch/i386/kernel/semaphore.c
*
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
* and Michel Lespinasse <walken@google.com>
*
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*/
#include <linux/rwsem.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/sched/signal.h>
#include <linux/sched/rt.h>
#include <linux/sched/wake_q.h>
#include <linux/sched/debug.h>
#include <linux/osq_lock.h>
#include "rwsem.h"
/*
* Guide to the rw_semaphore's count field for common values.
* (32-bit case illustrated, similar for 64-bit)
*
* 0x0000000X (1) X readers active or attempting lock, no writer waiting
* X = #active_readers + #readers attempting to lock
* (X*ACTIVE_BIAS)
*
* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
* attempting to read lock or write lock.
*
* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
* X = #active readers + # readers attempting lock
* (X*ACTIVE_BIAS + WAITING_BIAS)
* (2) 1 writer attempting lock, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
* (3) 1 writer active, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
*
* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
* (WAITING_BIAS + ACTIVE_BIAS)
* (2) 1 writer active or attempting lock, no waiters for lock
* (ACTIVE_WRITE_BIAS)
*
* 0xffff0000 (1) There are writers or readers queued but none active
* or in the process of attempting lock.
* (WAITING_BIAS)
* Note: writer can attempt to steal lock for this count by adding
* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
*
* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
*
* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
* the count becomes more than 0 for successful lock acquisition,
* i.e. the case where there are only readers or nobody has lock.
* (1st and 2nd case above).
*
* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
* acquisition (i.e. nobody else has lock or attempts lock). If
* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
* are only waiters but none active (5th case above), and attempt to
* steal the lock.
*
*/
/*
* Initialize an rwsem:
*/
void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* Make sure we are not reinitializing a held semaphore:
*/
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
sem->owner = NULL;
osq_lock_init(&sem->osq);
#endif
}
EXPORT_SYMBOL(__init_rwsem);
enum rwsem_waiter_type {
RWSEM_WAITING_FOR_WRITE,
RWSEM_WAITING_FOR_READ
};
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
};
enum rwsem_wake_type {
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
RWSEM_WAKE_READERS, /* Wake readers only */
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
};
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then:
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
* - there must be someone on the queue
* - the wait_lock must be held by the caller
* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
* to actually wakeup the blocked task(s) and drop the reference count,
* preferably when the wait_lock is released
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only marked woken if downgrading is false
*/
static void __rwsem_mark_wake(struct rw_semaphore *sem,
enum rwsem_wake_type wake_type,
struct wake_q_head *wake_q)
{
struct rwsem_waiter *waiter, *tmp;
long oldcount, woken = 0, adjustment = 0;
/*
* Take a peek at the queue head waiter such that we can determine
* the wakeup(s) to perform.
*/
waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type == RWSEM_WAKE_ANY) {
/*
* Mark writer at the front of the queue for wakeup.
* Until the task is actually later awoken later by
* the caller, other writers are able to steal it.
* Readers, on the other hand, will block as they
* will notice the queued writer.
*/
wake_q_add(wake_q, waiter->task);
}
return;
}
/*
* Writers might steal the lock before we grant it to the next reader.
* We prefer to do the first reader grant before counting readers
* so we can bail out early if a writer stole the lock.
*/
if (wake_type != RWSEM_WAKE_READ_OWNED) {
adjustment = RWSEM_ACTIVE_READ_BIAS;
try_reader_grant:
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
/*
* If the count is still less than RWSEM_WAITING_BIAS
* after removing the adjustment, it is assumed that
* a writer has stolen the lock. We have to undo our
* reader grant.
*/
if (atomic_long_add_return(-adjustment, &sem->count) <
RWSEM_WAITING_BIAS)
return;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;
}
/*
* It is not really necessary to set it to reader-owned here,
* but it gives the spinners an early indication that the
* readers now have the lock.
*/
rwsem_set_reader_owned(sem);
}
/*
* Grant an infinite number of read locks to the readers at the front
* of the queue. We know that woken will be at least 1 as we accounted
* for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
*/
list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
struct task_struct *tsk;
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
break;
woken++;
tsk = waiter->task;
wake_q_add(wake_q, tsk);
list_del(&waiter->list);
/*
* Ensure that the last operation is setting the reader
* waiter to nil such that rwsem_down_read_failed() cannot
* race with do_exit() by always holding a reference count
* to the task to wakeup.
*/
smp_store_release(&waiter->task, NULL);
}
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
if (list_empty(&sem->wait_list)) {
/* hit end of list above */
adjustment -= RWSEM_WAITING_BIAS;
}
if (adjustment)
atomic_long_add(adjustment, &sem->count);
}
/*
* Wait for the read lock to be granted
*/
static inline struct rw_semaphore __sched *
__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
{
long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list))
adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
count = atomic_long_add_return(adjustment, &sem->count);
/*
* If there are no active locks, wake the front queued process(es).
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
if (count == RWSEM_WAITING_BIAS ||
(count > RWSEM_WAITING_BIAS &&
adjustment != -RWSEM_ACTIVE_READ_BIAS))
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
/* wait to be given the lock */
while (true) {
set_current_state(state);
if (!waiter.task)
break;
if (signal_pending_state(state, current)) {
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task)
goto out_nolock;
raw_spin_unlock_irq(&sem->wait_lock);
break;
}
schedule();
}
__set_current_state(TASK_RUNNING);
return sem;
out_nolock:
list_del(&waiter.list);
if (list_empty(&sem->wait_list))
atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
return ERR_PTR(-EINTR);
}
__visible struct rw_semaphore * __sched
rwsem_down_read_failed(struct rw_semaphore *sem)
{
return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(rwsem_down_read_failed);
__visible struct rw_semaphore * __sched
rwsem_down_read_failed_killable(struct rw_semaphore *sem)
{
return __rwsem_down_read_failed_common(sem, TASK_KILLABLE);
}
EXPORT_SYMBOL(rwsem_down_read_failed_killable);
/*
* This function must be called with the sem->wait_lock held to prevent
* race conditions between checking the rwsem wait list and setting the
* sem->count accordingly.
*/
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
/*
* Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
*/
if (count != RWSEM_WAITING_BIAS)
return false;
/*
* Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
* are other tasks on the wait list, we need to add on WAITING_BIAS.
*/
count = list_is_singular(&sem->wait_list) ?
RWSEM_ACTIVE_WRITE_BIAS :
RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
== RWSEM_WAITING_BIAS) {
rwsem_set_owner(sem);
return true;
}
return false;
}
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
* Try to acquire write lock before the writer has been put on wait queue.
*/
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
long old, count = atomic_long_read(&sem->count);
while (true) {
if (!(count == 0 || count == RWSEM_WAITING_BIAS))
return false;
old = atomic_long_cmpxchg_acquire(&sem->count, count,
count + RWSEM_ACTIVE_WRITE_BIAS);
if (old == count) {
rwsem_set_owner(sem);
return true;
}
count = old;
}
}
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner;
bool ret = true;
if (need_resched())
return false;
rcu_read_lock();
owner = READ_ONCE(sem->owner);
if (!rwsem_owner_is_writer(owner)) {
/*
* Don't spin if the rwsem is readers owned.
*/
ret = !rwsem_owner_is_reader(owner);
goto done;
}
/*
* As lock holder preemption issue, we both skip spinning if task is not
* on cpu or its cpu is preempted
*/
ret = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
done:
rcu_read_unlock();
return ret;
}
/*
* Return true only if we can still spin on the owner field of the rwsem.
*/
static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner = READ_ONCE(sem->owner);
if (!rwsem_owner_is_writer(owner))
goto out;
rcu_read_lock();
while (sem->owner == owner) {
/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking sem->owner still matches owner, if that fails,
* owner might point to free()d memory, if it still matches,
* the rcu_read_lock() ensures the memory stays valid.
*/
barrier();
/*
* abort spinning when need_resched or owner is not running or
* owner's cpu is preempted.
*/
if (!owner->on_cpu || need_resched() ||
vcpu_is_preempted(task_cpu(owner))) {
rcu_read_unlock();
return false;
}
cpu_relax();
}
rcu_read_unlock();
out:
/*
* If there is a new owner or the owner is not set, we continue
* spinning.
*/
return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
}
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
bool taken = false;
preempt_disable();
/* sem->wait_lock should not be held when doing optimistic spinning */
if (!rwsem_can_spin_on_owner(sem))
goto done;
if (!osq_lock(&sem->osq))
goto done;
/*
* Optimistically spin on the owner field and attempt to acquire the
* lock whenever the owner changes. Spinning will be stopped when:
* 1) the owning writer isn't running; or
* 2) readers own the lock as we can't determine if they are
* actively running or not.
*/
while (rwsem_spin_on_owner(sem)) {
/*
* Try to acquire the lock
*/
if (rwsem_try_write_lock_unqueued(sem)) {
taken = true;
break;
}
/*
* When there's no owner, we might have preempted between the
* owner acquiring the lock and setting the owner field. If
* we're an RT task that will live-lock because we won't let
* the owner complete.
*/
if (!sem->owner && (need_resched() || rt_task(current)))
break;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
cpu_relax();
}
osq_unlock(&sem->osq);
done:
preempt_enable();
return taken;
}
/*
* Return true if the rwsem has active spinner
*/
static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
{
return osq_is_locked(&sem->osq);
}
#else
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
return false;
}
static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
{
return false;
}
#endif
/*
* Wait until we successfully acquire the write lock
*/
static inline struct rw_semaphore *
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
{
long count;
bool waiting = true; /* any queued threads before us */
struct rwsem_waiter waiter;
struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
/* undo write bias from down_write operation, stop active locking */
count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
/* do optimistic spinning and steal lock if possible */
if (rwsem_optimistic_spin(sem))
return sem;
/*
* Optimistic spinning failed, proceed to the slowpath
* and block until we can acquire the sem.
*/
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
raw_spin_lock_irq(&sem->wait_lock);
/* account for this before adding a new element to the list */
if (list_empty(&sem->wait_list))
waiting = false;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
if (waiting) {
count = atomic_long_read(&sem->count);
/*
* If there were already threads queued before us and there are
* no active writers, the lock must be read owned; so we try to
* wake any read locks that were queued ahead of us.
*/
if (count > RWSEM_WAITING_BIAS) {
__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
/*
* The wakeup is normally called _after_ the wait_lock
* is released, but given that we are proactively waking
* readers we can deal with the wake_q overhead as it is
* similar to releasing and taking the wait_lock again
* for attempting rwsem_try_write_lock().
*/
wake_up_q(&wake_q);
/*
* Reinitialize wake_q after use.
*/
wake_q_init(&wake_q);
}
} else
count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
/* wait until we successfully acquire the lock */
set_current_state(state);
while (true) {
if (rwsem_try_write_lock(count, sem))
break;
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
do {
if (signal_pending_state(state, current))
goto out_nolock;
schedule();
set_current_state(state);
} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock);
}
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
return ret;
out_nolock:
__set_current_state(TASK_RUNNING);
raw_spin_lock_irq(&sem->wait_lock);
list_del(&waiter.list);
if (list_empty(&sem->wait_list))
atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
else
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
return ERR_PTR(-EINTR);
}
__visible struct rw_semaphore * __sched
rwsem_down_write_failed(struct rw_semaphore *sem)
{
return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(rwsem_down_write_failed);
__visible struct rw_semaphore * __sched
rwsem_down_write_failed_killable(struct rw_semaphore *sem)
{
return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
}
EXPORT_SYMBOL(rwsem_down_write_failed_killable);
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
*/
__visible
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
/*
* __rwsem_down_write_failed_common(sem)
* rwsem_optimistic_spin(sem)
* osq_unlock(sem->osq)
* ...
* atomic_long_add_return(&sem->count)
*
* - VS -
*
* __up_write()
* if (atomic_long_sub_return_release(&sem->count) < 0)
* rwsem_wake(sem)
* osq_is_locked(&sem->osq)
*
* And __up_write() must observe !osq_is_locked() when it observes the
* atomic_long_add_return() in order to not miss a wakeup.
*
* This boils down to:
*
* [S.rel] X = 1 [RmW] r0 = (Y += 0)
* MB RMB
* [RmW] Y += 1 [L] r1 = X
*
* exists (r0=1 /\ r1=0)
*/
smp_rmb();
/*
* If a spinner is present, it is not necessary to do the wakeup.
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the
* unlock operation.
*
* spinning writer up_write/up_read caller
* --------------- -----------------------
* [S] osq_unlock() [L] osq
* MB RMB
* [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
*
* Here, it is important to make sure that there won't be a missed
* wakeup while the rwsem is free and the only spinning writer goes
* to sleep without taking the rwsem. Even when the spinning writer
* is just going to break out of the waiting loop, it will still do
* a trylock in rwsem_down_write_failed() before sleeping. IOW, if
* rwsem_has_spinner() is true, it will guarantee at least one
* trylock attempt on the rwsem later on.
*/
if (rwsem_has_spinner(sem)) {
/*
* The smp_rmb() here is to make sure that the spinner
* state is consulted before reading the wait_lock.
*/
smp_rmb();
if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
return sem;
goto locked;
}
raw_spin_lock_irqsave(&sem->wait_lock, flags);
locked:
if (!list_empty(&sem->wait_list))
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
wake_up_q(&wake_q);
return sem;
}
EXPORT_SYMBOL(rwsem_wake);
/*
* downgrade a write lock into a read lock
* - caller incremented waiting part of count and discovered it still negative
* - just wake up any readers at the front of the queue
*/
__visible
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
raw_spin_lock_irqsave(&sem->wait_lock, flags);
if (!list_empty(&sem->wait_list))
__rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
wake_up_q(&wake_q);
return sem;
}
EXPORT_SYMBOL(rwsem_downgrade_wake);