The following commit has been merged in the linux branch: commit d58e6576b0deec6f0b9ff8450fe282da18c50883 Author: Thomas Gleixner tglx@linutronix.de Date: Tue Oct 13 20:40:43 2009 +0200

futex: Handle spurious wake up

The futex code does not handle spurious wake up in futex_wait and futex_wait_requeue_pi.

The code assumes that any wake up which was not caused by futex_wake / requeue or by a timeout was caused by a signal wake up and returns one of the syscall restart error codes.

In case of a spurious wake up the signal delivery code which deals with the restart error codes is not invoked and we return that error code to user space. That causes applications which actually check the return codes to fail. Blaise reported that on preempt-rt a python test program run into a exception trap. -rt exposed that due to a built in spurious wake up accelerator :)

Solve this by checking signal_pending(current) in the wake up path and handle the spurious wake up case w/o returning to user space.

Reported-by: Blaise Gassend blaise@willowgarage.com Debugged-by: Darren Hart dvhltc@us.ibm.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: stable@kernel.org LKML-Reference: <new-submission>

diff --git a/kernel/futex.c b/kernel/futex.c index 4949d33..5c88839 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1791,6 +1791,7 @@ static int futex_wait(u32 __user *uaddr, int fshared, current->timer_slack_ns); }

+retry: /* Prepare to wait on uaddr. */ ret = futex_wait_setup(uaddr, val, fshared, &q, &hb); if (ret) @@ -1808,9 +1809,14 @@ static int futex_wait(u32 __user *uaddr, int fshared, goto out_put_key;

/* - * We expect signal_pending(current), but another thread may - * have handled it for us already. + * We expect signal_pending(current), but we might be the + * victim of a spurious wakeup as well. */ + if (!signal_pending(current)) { + put_futex_key(fshared, &q.key); + goto retry; + } + ret = -ERESTARTSYS; if (!abs_time) goto out_put_key; @@ -2118,9 +2124,11 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb, */ plist_del(&q->list, &q->list.plist);

+ /* Handle spurious wakeups gracefully */ + ret = -EAGAIN; if (timeout && !timeout->task) ret = -ETIMEDOUT; - else + else if (signal_pending(current)) ret = -ERESTARTNOINTR; } return ret; @@ -2198,6 +2206,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared, debug_rt_mutex_init_waiter(&rt_waiter); rt_waiter.task = NULL;

+retry: key2 = FUTEX_KEY_INIT; ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE); if (unlikely(ret != 0)) @@ -2292,6 +2301,9 @@ out_put_keys: out_key2: put_futex_key(fshared, &key2);

+ /* Spurious wakeup ? */ + if (ret == -EAGAIN) + goto retry; out: if (to) { hrtimer_cancel(&to->timer);