Techwizz/kernel-fxtec-pro1x
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

ipc/sem.c: update/correct memory barriers

Browse source 
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
ipc/sem.c.

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: Manfred Spraul <manfred@colorfullife.com>
Reported-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Kirill Tkhai <ktkhai@parallels.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: <stable@vger.kernel.org>	[3.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Manfred Spraul 2015-08-14 15:35:10 -07:00 committed by Linus Torvalds
parent 7f6bf39bbd
commit 3ed1f8a99d
1 changed files with 14 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

18
ipc/sem.c
View file

@ -252,6 +252,16 @@ static void sem_rcu_free(struct rcu_head *head)
ipc_rcu_free(head); ipc_rcu_free(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. * Wait until all currently ongoing simple ops have completed.
* Caller must own sem_perm.lock. * Caller must own sem_perm.lock.
@ -275,6 +285,7 @@ static void sem_wait_array(struct sem_array *sma)
sem = sma->sem_base + i; sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock); spin_unlock_wait(&sem->lock);
} }
ipc_smp_acquire__after_spin_is_unlocked();
} }
/* /*
@ -327,13 +338,12 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
/* Then check that the global lock is free */ /* Then check that the global lock is free */
if (!spin_is_locked(&sma->sem_perm.lock)) { if (!spin_is_locked(&sma->sem_perm.lock)) {
/* /*
* The ipc object lock check must be visible on all * We need a memory barrier with acquire semantics,
* cores before rechecking the complex count. Otherwise * otherwise we can race with another thread that does:
* we can race with another thread that does:
* complex_count++; * complex_count++;
* spin_unlock(sem_perm.lock); * spin_unlock(sem_perm.lock);
*/ */
smp_rmb(); ipc_smp_acquire__after_spin_is_unlocked();
/* /*
* Now repeat the test of complex_count: * Now repeat the test of complex_count: