Messages in this thread Patch in this message | | | Date | Fri, 10 Apr 2015 18:11:35 +0200 | From | Sebastian Andrzej Siewior <> | Subject | [PATCH 2/3 v2] futex: avoid double wake up in futex_wake() on -RT |
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futex_wake() wakes the waiter while holding the hb->lock. The waiter does not take the hb->lock and can leave the kernel. However the next operation the same futex operation will point to the same hb->lock and we will see a small dance around the lock including prio-boosting and context switch:
low prio task FUTEX_WAKE on high prio | ft-1489 [000] ....1.. 81.167501: sys_enter: sys_futex (8049f60, 1, 1, 0, 0, 0) | ft-1489 [000] dN..311 81.167504: sched_wakeup: pid=1490 prio=94 | ft-1489 [000] d...311 81.167520: sched_switch: prev_pid=1489 prev_prio=120 prev_state=R+ ==> next_pid=1490 next_prio=94 | ft-1490 [000] ....1.. 81.167522: sys_exit: sys_futex = 0
prio task FUTEX_WAKE on low prio | ft-1490 [000] ....1.. 81.167528: sys_enter: sys_futex (8049f5c, 1, 1, 0, 0, 0) | ft-1490 [000] ....1.. 81.167530: sys_exit: sys_futex = 0
prio task waits FUTEX_WAIT, hb->lock still owned by low prio task | ft-1490 [000] ....1.. 81.167534: sys_enter: sys_futex (8049f60, 0, 1, 0, 0, 0) | ft-1490 [000] d...411 81.167895: sched_pi_setprio: pid=1489 oldprio=120 newprio=94 | ft-1490 [000] d...311 81.167901: sched_switch: prev_pid=1490 prev_prio=94 prev_state=D ==> next_pid=1489 next_prio=94 | ft-1489 [000] d...411 81.167910: sched_wakeup: pid=1490 prio=94 | ft-1489 [000] d...311 81.167912: sched_pi_setprio: pid=1489 oldprio=94 newprio=120 | ft-1489 [000] d...311 81.167915: sched_switch: prev_pid=1489 prev_prio=120 prev_state=R+ ==> next_pid=1490 next_prio=94 | ft-1490 [000] d...3.. 81.167922: sched_switch: prev_pid=1490 prev_prio=94 prev_state=S ==> next_pid=1489 next_prio=120 | ft-1489 [000] ....1.. 81.167924: sys_exit: sys_futex = 1
This patch delays the wakeup of the process untill the hb->lock is dropped to avoid boosting + context switch to obtain the lock.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- v1…v2: - update patch description - move the comment to __wake_futex() - move the wakeup before the out_put_key label in futex_wake()
kernel/futex.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index b38abe3573a8..658f4d05cd6f 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1092,12 +1092,12 @@ static void __unqueue_futex(struct futex_q *q) * The hash bucket lock must be held when this is called. * Afterwards, the futex_q must not be accessed. */ -static void wake_futex(struct futex_q *q) +static struct task_struct *__wake_futex(struct futex_q *q) { struct task_struct *p = q->task; if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n")) - return; + return NULL; /* * We set q->lock_ptr = NULL _before_ we wake up the task. If @@ -1117,6 +1117,15 @@ static void wake_futex(struct futex_q *q) */ smp_wmb(); q->lock_ptr = NULL; + return p; +} + +static void wake_futex(struct futex_q *q) +{ + struct task_struct *p = __wake_futex(q); + + if (!p) + return; wake_up_state(p, TASK_NORMAL); put_task_struct(p); @@ -1228,6 +1237,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) struct futex_hash_bucket *hb; struct futex_q *this, *next; union futex_key key = FUTEX_KEY_INIT; + struct task_struct *waiter = NULL; int ret; if (!bitset) @@ -1256,13 +1266,21 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) if (!(this->bitset & bitset)) continue; - wake_futex(this); + if (nr_wake == 1) + waiter = __wake_futex(this); + else + wake_futex(this); if (++ret >= nr_wake) break; } } spin_unlock(&hb->lock); + if (waiter) { + wake_up_state(waiter, TASK_NORMAL); + put_task_struct(waiter); + } + out_put_key: put_futex_key(&key); out: -- 2.1.4
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