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ipc/sem: avoid idr tree lookup for interrupted semop

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We can avoid the idr tree lookup (albeit possibly avoiding
idr_find_fast()) when being awoken in EINTR, as the semid will not
change in this context while blocked.  Use the sma pointer directly and
take the sem_lock, then re-check for RMID races.  We continue to
re-check the queue.status with the lock held such that we can detect
situations where we where are dealing with a spurious wakeup but another
task that holds the sem_lock updated the queue.status while we were
spinning for it.  Once we take the lock it obviously won't change again.

Being the only caller, get rid of sem_obtain_lock() altogether.

Link: http://lkml.kernel.org/r/1478708774-28826-3-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Davidlohr Bueso 2016-12-14 15:06:49 -08:00 committed by Linus Torvalds
parent b5fa01a22e
commit 370b262c89
1 changed files with 5 additions and 32 deletions
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37
ipc/sem.c
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@ -414,29 +414,6 @@ static inline void sem_unlock(struct sem_array *sma, int locknum)
*
* The caller holds the RCU read lock.
*/
static inline struct sem_array *sem_obtain_lock(struct ipc_namespace *ns,
int id, struct sembuf *sops, int nsops, int *locknum)
{
struct kern_ipc_perm *ipcp;
struct sem_array *sma;
ipcp = ipc_obtain_object_idr(&sem_ids(ns), id);
if (IS_ERR(ipcp))
return ERR_CAST(ipcp);
sma = container_of(ipcp, struct sem_array, sem_perm);
*locknum = sem_lock(sma, sops, nsops);
/* ipc_rmid() may have already freed the ID while sem_lock
* was spinning: verify that the structure is still valid
*/
if (ipc_valid_object(ipcp))
return container_of(ipcp, struct sem_array, sem_perm);
sem_unlock(sma, *locknum);
return ERR_PTR(-EINVAL);
}
static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
{
struct kern_ipc_perm *ipcp = ipc_obtain_object_idr(&sem_ids(ns), id);
@ -2000,16 +1977,12 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
}
rcu_read_lock();
sma = sem_obtain_lock(ns, semid, sops, nsops, &locknum);
error = READ_ONCE(queue.status);
sem_lock(sma, sops, nsops);
/*
* Array removed? If yes, leave without sem_unlock().
*/
if (IS_ERR(sma)) {
rcu_read_unlock();
goto out_free;
}
if (!ipc_valid_object(&sma->sem_perm))
goto out_unlock_free;
error = READ_ONCE(queue.status);
/*
* If queue.status != -EINTR we are woken up by another process.