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sched/swait: Switch to full exclusive mode

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Linus noted that swait basically implements exclusive mode -- because
swake_up() only wakes a single waiter. And because of that it should
take care to properly deal with the interruptible case.

In short, the problem is that swake_up() can race with a signal. In
this this case it is possible the swake_up() 'wakes' the waiter that
is already on the way out because it just got a signal and the wakeup
gets lost.

The normal wait code is very careful and avoids this situation, make
sure we do too.

Copy the exact exclusive semantics from wait.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: bigeasy@linutronix.de
Cc: oleg@redhat.com
Cc: paulmck@linux.vnet.ibm.com
Cc: pbonzini@redhat.com
Link: https://lkml.kernel.org/r/20180612083909.209762413@infradead.org
This commit is contained in:
Peter Zijlstra 2018-06-12 10:34:51 +02:00 committed by Thomas Gleixner
parent 6519750210
commit 0abf17bc77
2 changed files with 23 additions and 10 deletions
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11
include/linux/swait.h
View file

@ -38,8 +38,8 @@
* all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
* sleeper state. * sleeper state.
* *
* - the exclusive mode; because this requires preserving the list order * - the !exclusive mode; because that leads to O(n) wakeups, everything is
* and this is hard. * exclusive.
* *
* - custom wake callback functions; because you cannot give any guarantees * - custom wake callback functions; because you cannot give any guarantees
* about random code. This also allows swait to be used in RT, such that * about random code. This also allows swait to be used in RT, such that
@ -167,9 +167,10 @@ extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queu
extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait); extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait); extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
/* as per ___wait_event() but for swait, therefore "exclusive == 0" */ /* as per ___wait_event() but for swait, therefore "exclusive == 1" */
#define ___swait_event(wq, condition, state, ret, cmd) \ #define ___swait_event(wq, condition, state, ret, cmd) \
({ \ ({ \
__label__ __out; \
struct swait_queue __wait; \ struct swait_queue __wait; \
long __ret = ret; \ long __ret = ret; \
\ \
@ -182,13 +183,13 @@ extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
\ \
if (___wait_is_interruptible(state) && __int) { \ if (___wait_is_interruptible(state) && __int) { \
__ret = __int; \ __ret = __int; \
break; \ goto __out; \
} \ } \
\ \
cmd; \ cmd; \
} \ } \
finish_swait(&wq, &__wait); \ finish_swait(&wq, &__wait); \
__ret; \ __out: __ret; \
}) })
#define __swait_event(wq, condition) \ #define __swait_event(wq, condition) \

22
kernel/sched/swait.c
View file

@ -73,7 +73,7 @@ static void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *w
{ {
wait->task = current; wait->task = current;
if (list_empty(&wait->task_list)) if (list_empty(&wait->task_list))
list_add(&wait->task_list, &q->task_list); list_add_tail(&wait->task_list, &q->task_list);
} }
void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state) void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
@ -89,12 +89,24 @@ EXPORT_SYMBOL(prepare_to_swait);
long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state) long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state)
{ {
if (signal_pending_state(state, current)) unsigned long flags;
return -ERESTARTSYS; long ret = 0;
prepare_to_swait(q, wait, state); raw_spin_lock_irqsave(&q->lock, flags);
if (unlikely(signal_pending_state(state, current))) {
/*
* See prepare_to_wait_event(). TL;DR, subsequent swake_up()
* must not see us.
*/
list_del_init(&wait->task_list);
ret = -ERESTARTSYS;
} else {
__prepare_to_swait(q, wait);
set_current_state(state);
}
raw_spin_unlock_irqrestore(&q->lock, flags);
return 0; return ret;
} }
EXPORT_SYMBOL(prepare_to_swait_event); EXPORT_SYMBOL(prepare_to_swait_event);