On Fri, 30 Sep 2016 10:39:14 +0200
Sebastian Andrzej Siewior <bige...@linutronix.de> wrote:
> The sequence:
> T1 holds futex
> T2 blocks on futex and boosts T1
> T1 unlocks futex and holds hb->lock
> T1 unlocks rt mutex, so T1 has no more pi waiters
> T3 blocks on hb->lock and adds itself to the pi waiters list of T1
> T1 unlocks hb->lock and deboosts itself
> T4 preempts T1 so the wakeup of T2 gets delayed
>
> As a workaround I attempt here do unlock the hb->lock without a deboost
> and perform the deboost after the wake up of the waiter.
>
> Cc: stable...@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> ---
> include/linux/spinlock.h | 6 +++++
> include/linux/spinlock_rt.h | 2 ++
> kernel/futex.c | 2 +-
> kernel/locking/rtmutex.c | 53
> +++++++++++++++++++++++++++++++++++++++------
> 4 files changed, 55 insertions(+), 8 deletions(-)
>
This looks awfully complex. Would something as simple as this work?
What harm can happen by moving the holding of the lock after the
wakeups for RT?
-- Steve
diff --git a/kernel/futex.c b/kernel/futex.c
index 2d572ed..bb900bd 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1347,9 +1347,14 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval,
struct futex_q *this,
* deboost first (and lose our higher priority), then the task might get
* scheduled away before the wake up can take place.
*/
+#ifndef CONFIG_PREEMPT_RT_FULL
spin_unlock(&hb->lock);
+#endif
wake_up_q(&wake_q);
wake_up_q_sleeper(&wake_sleeper_q);
+#ifdef CONFIG_PREEMPT_RT_FULL
+ spin_unlock(&hb->lock);
+#endif
if (deboost)
rt_mutex_adjust_prio(current);
