From 61ada528dea028331e99e8ceaed87c683ad25de2 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 24 Sep 2014 10:18:47 +0200 Subject: sched/wait: Provide infrastructure to deal with nested blocking There are a few places that call blocking primitives from wait loops, provide infrastructure to support this without the typical task_struct::state collision. We record the wakeup in wait_queue_t::flags which leaves task_struct::state free to be used by others. Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Oleg Nesterov Cc: tglx@linutronix.de Cc: ilya.dryomov@inktank.com Cc: umgwanakikbuti@gmail.com Cc: Linus Torvalds Link: http://lkml.kernel.org/r/20140924082242.051202318@infradead.org Signed-off-by: Ingo Molnar --- include/linux/wait.h | 7 +++++- kernel/sched/wait.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 67 insertions(+), 1 deletion(-) diff --git a/include/linux/wait.h b/include/linux/wait.h index e4a8eb9312ea..fc0e99395fbb 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -13,9 +13,12 @@ typedef struct __wait_queue wait_queue_t; typedef int (*wait_queue_func_t)(wait_queue_t *wait, unsigned mode, int flags, void *key); int default_wake_function(wait_queue_t *wait, unsigned mode, int flags, void *key); +/* __wait_queue::flags */ +#define WQ_FLAG_EXCLUSIVE 0x01 +#define WQ_FLAG_WOKEN 0x02 + struct __wait_queue { unsigned int flags; -#define WQ_FLAG_EXCLUSIVE 0x01 void *private; wait_queue_func_t func; struct list_head task_list; @@ -830,6 +833,8 @@ void prepare_to_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait, int sta long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state); void finish_wait(wait_queue_head_t *q, wait_queue_t *wait); void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait, unsigned int mode, void *key); +long wait_woken(wait_queue_t *wait, unsigned mode, long timeout); +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key); int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key); int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *key); diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c index 5a62915f47a8..4dae1885db6f 100644 --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -297,6 +297,67 @@ int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int sync, void * } EXPORT_SYMBOL(autoremove_wake_function); + +/* + * DEFINE_WAIT_FUNC(wait, woken_wake_func); + * + * add_wait_queue(&wq, &wait); + * for (;;) { + * if (condition) + * break; + * + * p->state = mode; condition = true; + * smp_mb(); // A smp_wmb(); // C + * if (!wait->flags & WQ_FLAG_WOKEN) wait->flags |= WQ_FLAG_WOKEN; + * schedule() try_to_wake_up(); + * p->state = TASK_RUNNING; ~~~~~~~~~~~~~~~~~~ + * wait->flags &= ~WQ_FLAG_WOKEN; condition = true; + * smp_mb() // B smp_wmb(); // C + * wait->flags |= WQ_FLAG_WOKEN; + * } + * remove_wait_queue(&wq, &wait); + * + */ +long wait_woken(wait_queue_t *wait, unsigned mode, long timeout) +{ + set_current_state(mode); /* A */ + /* + * The above implies an smp_mb(), which matches with the smp_wmb() from + * woken_wake_function() such that if we observe WQ_FLAG_WOKEN we must + * also observe all state before the wakeup. + */ + if (!(wait->flags & WQ_FLAG_WOKEN)) + timeout = schedule_timeout(timeout); + __set_current_state(TASK_RUNNING); + + /* + * The below implies an smp_mb(), it too pairs with the smp_wmb() from + * woken_wake_function() such that we must either observe the wait + * condition being true _OR_ WQ_FLAG_WOKEN such that we will not miss + * an event. + */ + set_mb(wait->flags, wait->flags & ~WQ_FLAG_WOKEN); /* B */ + + return timeout; +} +EXPORT_SYMBOL(wait_woken); + +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key) +{ + /* + * Although this function is called under waitqueue lock, LOCK + * doesn't imply write barrier and the users expects write + * barrier semantics on wakeup functions. The following + * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up() + * and is paired with set_mb() in wait_woken(). + */ + smp_wmb(); /* C */ + wait->flags |= WQ_FLAG_WOKEN; + + return default_wake_function(wait, mode, sync, key); +} +EXPORT_SYMBOL(woken_wake_function); + int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg) { struct wait_bit_key *key = arg; -- cgit v1.2.3