From c8446b75be6f85b3d40066922876cb7adc948afb Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 30 Oct 2012 13:33:54 +0100 Subject: [PATCH] irq_work: Fix racy IRQ_WORK_BUSY flag setting The IRQ_WORK_BUSY flag is set right before we execute the work. Once this flag value is set, the work enters a claimable state again. So if we have specific data to compute in our work, we ensure it's either handled by another CPU or locally by enqueuing the work again. This state machine is guanranteed by atomic operations on the flags. So when we set IRQ_WORK_BUSY without using an xchg-like operation, we break this guarantee as in the following summarized scenario: CPU 1 CPU 2 ----- ----- (flags = 0) old_flags = flags; (flags = 0) cmpxchg(flags, old_flags, old_flags | IRQ_WORK_FLAGS) (flags = 3) [...] flags = IRQ_WORK_BUSY (flags = 2) func() (sees flags = 3) cmpxchg(flags, old_flags, old_flags | IRQ_WORK_FLAGS) (give up) cmpxchg(flags, 2, 0); (flags = 0) CPU 1 claims a work and executes it, so it sets IRQ_WORK_BUSY and the work is again in a claimable state. Now CPU 2 has new data to process and try to claim that work but it may see a stale value of the flags and think the work is still pending somewhere that will handle our data. This is because CPU 1 doesn't set IRQ_WORK_BUSY atomically. As a result, the data expected to be handle by CPU 2 won't get handled. To fix this, use xchg() to set IRQ_WORK_BUSY, this way we ensure the CPU 2 will see the correct value with cmpxchg() using the expected ordering. Changelog-heavily-inspired-by: Steven Rostedt Signed-off-by: Frederic Weisbecker Acked-by: Steven Rostedt Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Thomas Gleixner Cc: Andrew Morton Cc: Paul Gortmaker Cc: Anish Kumar --- kernel/irq_work.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/irq_work.c b/kernel/irq_work.c index 1588e3b2871b..57be1a6cd8da 100644 --- a/kernel/irq_work.c +++ b/kernel/irq_work.c @@ -119,8 +119,11 @@ void irq_work_run(void) /* * Clear the PENDING bit, after this point the @work * can be re-used. + * Make it immediately visible so that other CPUs trying + * to claim that work don't rely on us to handle their data + * while we are in the middle of the func. */ - work->flags = IRQ_WORK_BUSY; + xchg(&work->flags, IRQ_WORK_BUSY); work->func(work); /* * Clear the BUSY bit and return to the free state if -- GitLab