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Commit 370b262c authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Linus Torvalds
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ipc/sem: avoid idr tree lookup for interrupted semop 

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: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b5fa01a2

ipc/sem.c

+5 −32
Original line number Diff line number Diff line
@@ -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.