From 25269871db1ad0cbbaafd5098cbdb40c8db4ccb9 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Fri, 8 Nov 2019 17:08:56 +0100 Subject: [PATCH] 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 Cc: Linus Torvalds Cc: Paul E . McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Link: https://lkml.kernel.org/r/20191108160858.31665-3-frederic@kernel.org Signed-off-by: Ingo Molnar --- kernel/irq_work.c | 22 +++++++--------------- 1 file changed, 7 insertions(+), 15 deletions(-) diff --git a/kernel/irq_work.c b/kernel/irq_work.c index df0dbf4..255454a 100644 --- a/kernel/irq_work.c +++ b/kernel/irq_work.c @@ -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(); - } - + if (oflags & IRQ_WORK_PENDING) + return false; return true; } -- 2.7.4