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membarrier: Add an actual barrier before rseq_preempt()

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It seems that most RSEQ membarrier users will expect any stores done before
the membarrier() syscall to be visible to the target task(s).  While this
is extremely likely to be true in practice, nothing actually guarantees it
by a strict reading of the x86 manuals.  Rather than providing this
guarantee by accident and potentially causing a problem down the road, just
add an explicit barrier.

Fixes: 70216e18e5 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/d3e7197e034fa4852afcf370ca49c30496e58e40.1607058304.git.luto@kernel.org
This commit is contained in:
Andy Lutomirski 2020-12-03 21:07:04 -08:00 committed by Thomas Gleixner
parent a493d1ca1a
commit 2ecedd7569
1 changed files with 8 additions and 0 deletions
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8
kernel/sched/membarrier.c
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@ -40,6 +40,14 @@ static void ipi_mb(void *info)
static void ipi_rseq(void *info)
{
/*
* Ensure that all stores done by the calling thread are visible
* to the current task before the current task resumes. We could
* probably optimize this away on most architectures, but by the
* time we've already sent an IPI, the cost of the extra smp_mb()
* is negligible.
*/
smp_mb();
rseq_preempt(current);
}