irq_work: Slightly simplify IRQ_WORK_PENDING clearing

Instead of fetching the value of flags and perform an xchg() to clear
a bit, just use atomic_fetch_andnot() that is more suitable to do that
job in one operation while keeping the full ordering.
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/20191108160858.31665-4-frederic@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/irq_work.c

@@ -34,7 +34,7 @@ static bool irq_work_claim(struct irq_work *work)
 	oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->flags);
 	/*
 	 * If the work is already pending, no need to raise the IPI.
-	 * The pairing atomic_xchg() in irq_work_run() makes sure
+	 * The pairing atomic_fetch_andnot() in irq_work_run() makes sure
 	 * everything we did before is visible.
 	 */
 	if (oflags & IRQ_WORK_PENDING)
@@ -135,7 +135,6 @@ static void irq_work_run_list(struct llist_head *list)
 {
 	struct irq_work *work, *tmp;
 	struct llist_node *llnode;
-	int flags;
 
 	BUG_ON(!irqs_disabled());
 
@@ -144,6 +143,7 @@ static void irq_work_run_list(struct llist_head *list)
 
 	llnode = llist_del_all(list);
 	llist_for_each_entry_safe(work, tmp, llnode, llnode) {
+		int flags;
 		/*
 		 * Clear the PENDING bit, after this point the @work
 		 * can be re-used.
@@ -151,8 +151,7 @@ static void irq_work_run_list(struct llist_head *list)
 		 * to claim that work don't rely on us to handle their data
 		 * while we are in the middle of the func.
 		 */
-		flags = atomic_read(&work->flags) & ~IRQ_WORK_PENDING;
-		atomic_xchg(&work->flags, flags);
+		flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->flags);
 
 		work->func(work);
 		/*