path: root/kernel
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authorLinus Torvalds <>2014-04-08 15:30:07 -0700
committerLinus Torvalds <>2014-04-09 08:02:12 -0700
commit69cd9eba38867a493a043bb13eb9b33cad5f1a9a (patch)
treefec2fee0f3fc0c67d00dd07836b70fb230c11aac /kernel
parent75ff24fa52f0cc512ceee4c377632b91a3a80811 (diff)
futex: avoid race between requeue and wake
Jan Stancek reported: "pthread_cond_broadcast/4-1.c testcase from openposix testsuite (LTP) occasionally fails, because some threads fail to wake up. Testcase creates 5 threads, which are all waiting on same condition. Main thread then calls pthread_cond_broadcast() without holding mutex, which calls: futex(uaddr1, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647, uaddr2, ..) This immediately wakes up single thread A, which unlocks mutex and tries to wake up another thread: futex(uaddr2, FUTEX_WAKE_PRIVATE, 1) If thread A manages to call futex_wake() before any waiters are requeued for uaddr2, no other thread is woken up" The ordering constraints for the hash bucket waiter counting are that the waiter counts have to be incremented _before_ getting the spinlock (because the spinlock acts as part of the memory barrier), but the "requeue" operation didn't honor those rules, and nobody had even thought about that case. This fairly simple patch just increments the waiter count for the target hash bucket (hb2) when requeing a futex before taking the locks. It then decrements them again after releasing the lock - the code that actually moves the futex(es) between hash buckets will do the additional required waiter count housekeeping. Reported-and-tested-by: Jan Stancek <> Acked-by: Davidlohr Bueso <> Cc: Peter Zijlstra <> Cc: Thomas Gleixner <> Cc: # 3.14 Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'kernel')
1 files changed, 5 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 67dacaf..6801b37 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1452,6 +1452,7 @@ retry:
hb2 = hash_futex(&key2);
+ hb_waiters_inc(hb2);
double_lock_hb(hb1, hb2);
if (likely(cmpval != NULL)) {
@@ -1461,6 +1462,7 @@ retry_private:
if (unlikely(ret)) {
double_unlock_hb(hb1, hb2);
+ hb_waiters_dec(hb2);
ret = get_user(curval, uaddr1);
if (ret)
@@ -1510,6 +1512,7 @@ retry_private:
case -EFAULT:
double_unlock_hb(hb1, hb2);
+ hb_waiters_dec(hb2);
ret = fault_in_user_writeable(uaddr2);
@@ -1519,6 +1522,7 @@ retry_private:
case -EAGAIN:
/* The owner was exiting, try again. */
double_unlock_hb(hb1, hb2);
+ hb_waiters_dec(hb2);
@@ -1594,6 +1598,7 @@ retry_private:
double_unlock_hb(hb1, hb2);
+ hb_waiters_dec(hb2);
* drop_futex_key_refs() must be called outside the spinlocks. During