Messages in this thread | | | Date | Wed, 26 Feb 2014 15:05:54 -0800 | From | Kent Overstreet <> | Subject | Re: [PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags cpumask |
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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 <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);
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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