Messages in this thread Patch in this message | | | From | Alexander Gordeev <> | Subject | [PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags cpumask | Date | Thu, 6 Feb 2014 13:24:53 +0100 |
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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@redhat.com> Cc: Kent Overstreet <kmo@daterainc.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org> Acked-by: Kent Overstreet <kmo@daterainc.com> --- 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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