It seems to me 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.
Cc: stable@vger.kernel.org Reviewed-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Signed-off-by: Andy Lutomirski luto@kernel.org --- kernel/sched/membarrier.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index 5a40b3828ff2..6251d3d12abe 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -168,6 +168,14 @@ 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); }
On Thu, Dec 3, 2020 at 9:07 PM Andy Lutomirski luto@kernel.org wrote:
It seems to me 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.
Cc: stable@vger.kernel.org
Fixes: 2a36ab717e8f ("rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ")
which is new in 5.10, so it doesn't need cc:stable if it makes 5.10.
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 2ecedd7569080fd05c1a457e8af2165afecfa29f Gitweb: https://git.kernel.org/tip/2ecedd7569080fd05c1a457e8af2165afecfa29f Author: Andy Lutomirski luto@kernel.org AuthorDate: Thu, 03 Dec 2020 21:07:04 -08:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Wed, 09 Dec 2020 09:37:43 +01:00
membarrier: Add an actual barrier before rseq_preempt()
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: 70216e18e519 ("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/d3e7197e034fa4852afcf370ca49c30496e58e40.160705830...
--- kernel/sched/membarrier.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index e23e74d..7d98ef5 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -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); }
On Wed, Dec 9, 2020 at 12:42 AM tip-bot2 for Andy Lutomirski tip-bot2@linutronix.de wrote:
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 2ecedd7569080fd05c1a457e8af2165afecfa29f Gitweb: https://git.kernel.org/tip/2ecedd7569080fd05c1a457e8af2165afecfa29f Author: Andy Lutomirski luto@kernel.org AuthorDate: Thu, 03 Dec 2020 21:07:04 -08:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Wed, 09 Dec 2020 09:37:43 +01:00
membarrier: Add an actual barrier before rseq_preempt()
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: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Whoops, this got mangled on its way to tip. This should be:
Fixes: 2a36ab717e8f ("rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ")
and this patch does not need to be backported.
linux-stable-mirror@lists.linaro.org
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tip-bot2 for Andy Lutomirski