Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 2/2] irq_work: Fix racy IRQ_WORK_BUSY flag setting | Date | Tue, 30 Oct 2012 16:35:01 +0100 |
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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.
This is necessary because if we want to enqueue a work but we fail the claim, we want to ensure that the CPU where that work is still pending will see and handle the data we expected the work to compute.
This might not work as expected though because IRQ_WORK_BUSY isn't set atomically. By the time a CPU fails a work claim, this work may well have been already executed by the CPU where it was previously pending.
Due to the lack of appropriate memory barrier, the IRQ_WORK_BUSY flag value may not be visible by the CPU trying to claim while the work is executing, and that until we clear the busy bit in the work flags using cmpxchg() that implies the full barrier.
One solution could involve a full barrier between setting IRQ_WORK_BUSY flag and the work execution. This way we ensure that the work execution site sees the expected data and the claim site sees the IRQ_WORK_BUSY:
CPU 0 CPU 1
data = something flags = IRQ_WORK_BUSY smp_mb() (implicit with cmpxchg smp_mb() on flags in claim) execute_work (sees data from CPU 0) try to claim
As a shortcut, let's just use xchg() that implies a full memory barrier.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> --- kernel/irq_work.c | 7 +++++-- 1 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/irq_work.c b/kernel/irq_work.c index 764240a..ea79365 100644 --- a/kernel/irq_work.c +++ b/kernel/irq_work.c @@ -130,9 +130,12 @@ void irq_work_run(void) /* * Clear the PENDING bit, after this point the @work - * can be re-used. + * can be re-used. Use xchg to force ordering against + * data to process, such that if claiming fails on + * another CPU, we see and handle the data it wants + * us to process on the work. */ - 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 -- 1.7.5.4
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