lkml.org 
[lkml]   [2014]   [Feb]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags cpumask
Date
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


\
 
 \ /
  Last update: 2014-02-06 14:01    [W:0.097 / U:0.372 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site