[tip: irq/core] irq_work: Fix irq_work_claim() memory ordering

tip-bot2 for Frederic Weisbecker
2019-11-11T09:32:45Z

The following commit has been merged into the irq/core branch of tip:

Commit-ID: 25269871db1ad0cbbaafd5098cbdb40c8db4ccb9
Gitweb: https://git.kernel.org/tip/25269871db1ad0cbbaafd5098cbdb40c8db4ccb9
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Fri, 08 Nov 2019 17:08:56 +01:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 11 Nov 2019 09:03:31 +01:00

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>
---
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;
}