Commit 3ed1f8a9 authored by Manfred Spraul's avatar Manfred Spraul Committed by Linus Torvalds

ipc/sem.c: update/correct memory barriers

sem_lock() did not properly pair memory barriers:

!spin_is_locked() and spin_unlock_wait() are both only control barriers.
The code needs an acquire barrier, otherwise the cpu might perform read
operations before the lock test.

As no primitive exists inside <include/spinlock.h> and since it seems
noone wants another primitive, the code creates a local primitive within

With regards to -stable:

The change of sem_wait_array() is a bugfix, the change to sem_lock() is a
nop (just a preprocessor redefinition to improve the readability).  The
bugfix is necessary for all kernels that use sem_wait_array() (i.e.:
starting from 3.10).
Signed-off-by: default avatarManfred Spraul <>
Reported-by: default avatarOleg Nesterov <>
Acked-by: default avatarPeter Zijlstra (Intel) <>
Cc: "Paul E. McKenney" <>
Cc: Kirill Tkhai <>
Cc: Ingo Molnar <>
Cc: Josh Poimboeuf <>
Cc: Davidlohr Bueso <>
Cc: <>	[3.10+]
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 7f6bf39b
......@@ -252,6 +252,16 @@ static void sem_rcu_free(struct rcu_head *head)
* spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
* are only control barriers.
* The code must pair with spin_unlock(&sem->lock) or
* spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient.
* smp_rmb() is sufficient, as writes cannot pass the control barrier.
#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb()
* Wait until all currently ongoing simple ops have completed.
* Caller must own sem_perm.lock.
......@@ -275,6 +285,7 @@ static void sem_wait_array(struct sem_array *sma)
sem = sma->sem_base + i;
......@@ -327,13 +338,12 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
/* Then check that the global lock is free */
if (!spin_is_locked(&sma->sem_perm.lock)) {
* The ipc object lock check must be visible on all
* cores before rechecking the complex count. Otherwise
* we can race with another thread that does:
* We need a memory barrier with acquire semantics,
* otherwise we can race with another thread that does:
* complex_count++;
* spin_unlock(sem_perm.lock);
* Now repeat the test of complex_count:
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