[PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags cpumask

[Alexander Gordeev]

Function steal_tags() might miss a bit in cpus_have_tags due to
unsynchronized access from percpu_ida_free(). As result, function
percpu_ida_alloc() might enter unwakable sleep. This update adds
memory barriers to prevent the described scenario.

In fact, accesses to cpus_have_tags are fenced by prepare_to_wait()
and wake_up() calls at the moment and the aforementioned sequence
does not appear could hit. Nevertheless, explicit memory barriers
still seem justifiable.

Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxx>
Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: "Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx>
Acked-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>
---
 lib/percpu_ida.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 7be235f..fccfb28 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -68,6 +68,11 @@ static inline void steal_tags(struct percpu_ida *pool,
 	unsigned cpus_have_tags, cpu = pool->cpu_last_stolen;
 	struct percpu_ida_cpu *remote;
 
+	/*
+	 * Pairs with smp_wmb() in percpu_ida_free()
+	 */
+	smp_rmb();
+
 	for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
 	     cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2;
 	     cpus_have_tags--) {
@@ -237,8 +242,11 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
 	spin_unlock(&tags->lock);
 
 	if (nr_free == 1) {
-		cpumask_set_cpu(smp_processor_id(),
-				&pool->cpus_have_tags);
+		cpumask_set_cpu(smp_processor_id(), &pool->cpus_have_tags);
+		/*
+		 * Pairs with smp_rmb() in steal_tags()
+		 */
+		smp_wmb();
 		wake_up(&pool->wait);
 	}
 
-- 
1.7.7.6

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