Commit-ID: 0abf17bc7790dd0467ed0e38522242f23c5da1c4 Gitweb: https://git.kernel.org/tip/0abf17bc7790dd0467ed0e38522242f23c5da1c4 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Tue, 12 Jun 2018 10:34:51 +0200 Committer: Thomas Gleixner <t...@linutronix.de> CommitDate: Wed, 20 Jun 2018 11:35:56 +0200
sched/swait: Switch to full exclusive mode 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 <torva...@linux-foundation.org> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Acked-by: Linus Torvalds <torva...@linux-foundation.org> Cc: bige...@linutronix.de Cc: o...@redhat.com Cc: paul...@linux.vnet.ibm.com Cc: pbonz...@redhat.com Link: https://lkml.kernel.org/r/20180612083909.209762...@infradead.org --- include/linux/swait.h | 11 ++++++----- kernel/sched/swait.c | 22 +++++++++++++++++----- 2 files changed, 23 insertions(+), 10 deletions(-) diff --git a/include/linux/swait.h b/include/linux/swait.h index d6a5e949e4ca..dd032061112d 100644 --- a/include/linux/swait.h +++ b/include/linux/swait.h @@ -38,8 +38,8 @@ * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right * sleeper state. * - * - the exclusive mode; because this requires preserving the list order - * and this is hard. + * - the !exclusive mode; because that leads to O(n) wakeups, everything is + * exclusive. * * - custom wake callback functions; because you cannot give any guarantees * 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); -/* 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) \ ({ \ + __label__ __out; \ struct swait_queue __wait; \ 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) { \ __ret = __int; \ - break; \ + goto __out; \ } \ \ cmd; \ } \ finish_swait(&wq, &__wait); \ - __ret; \ +__out: __ret; \ }) #define __swait_event(wq, condition) \ diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c index e68bb1398b05..66890de93ee5 100644 --- a/kernel/sched/swait.c +++ b/kernel/sched/swait.c @@ -73,7 +73,7 @@ static void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *w { wait->task = current; 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) @@ -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) { - if (signal_pending_state(state, current)) - return -ERESTARTSYS; + unsigned long flags; + 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);
