Commit 25269871 authored Nov 08, 2019 by Frederic Weisbecker Committed by Ingo Molnar Nov 11, 2019

irq_work: Fix irq_work_claim() memory ordering



When irq_work_claim() finds IRQ_WORK_PENDING flag already set, we just
return and don't raise a new IPI. We expect the destination to see
and handle our latest updades thanks to the pairing atomic_xchg()
in irq_work_run_list().

But cmpxchg() doesn't guarantee a full memory barrier upon failure. So
it's possible that the destination misses our latest updates.

So use atomic_fetch_or() instead that is unconditionally fully ordered
and also performs exactly what we want here and simplify the code.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191108160858.31665-3-frederic@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 153bedba

kernel/irq_work.c

+7 −15

Original line number	Diff line number	Diff line
		@@ -29,24 +29,16 @@ static DEFINE_PER_CPU(struct llist_head, lazy_list);
		*/
		static bool irq_work_claim(struct irq_work *work)
		{
		int flags, oflags, nflags;
		int oflags;

		oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->flags);
		/*
		* Start with our best wish as a premise but only trust any
		* flag value after cmpxchg() result.
		* If the work is already pending, no need to raise the IPI.
		* The pairing atomic_xchg() in irq_work_run() makes sure
		* everything we did before is visible.
		*/
		flags = atomic_read(&work->flags) & ~IRQ_WORK_PENDING;
		for (;;) {
		nflags = flags \| IRQ_WORK_CLAIMED;
		oflags = atomic_cmpxchg(&work->flags, flags, nflags);
		if (oflags == flags)
		break;
		if (oflags & IRQ_WORK_PENDING)
		return false;
		flags = oflags;
		cpu_relax();
		}

		return true;
		}

Admin message