On Thu, Feb 06, 2014 at 01:24:53PM +0100, Alexander Gordeev wrote:
> 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 <agord...@redhat.com>
> Cc: Kent Overstreet <k...@daterainc.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Jens Axboe <ax...@kernel.dk>
> Cc: "Nicholas A. Bellinger" <n...@linux-iscsi.org>
> Acked-by: Kent Overstreet <k...@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);
I think I'm nacking this - there's a lot of code in the kernel that relies on
the fact that prepare_to_wait)/wake_up() do the appropriate fences, we really
shouldn't be adding to the barriers those do.
If you can come up with some other reason we need the barriers I'll reconsider.
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