Re: [PATCH] ipc/sem.c: Update/correct memory barriers

From: Peter Zijlstra
Date: Mon Aug 10 2015 - 04:16:11 EST

On Sun, Aug 09, 2015 at 07:55:39PM +0200, Manfred Spraul wrote:
> 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).
> Andrew: Could you include it into your tree and forward it?
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
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