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completion: Use simple wait queues

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completion uses a wait_queue_head_t to enqueue waiters.

wait_queue_head_t contains a spinlock_t to protect the list of waiters
which excludes it from being used in truly atomic context on a PREEMPT_RT
enabled kernel.

The spinlock in the wait queue head cannot be replaced by a raw_spinlock
because:

  - wait queues can have custom wakeup callbacks, which acquire other
    spinlock_t locks and have potentially long execution times

  - wake_up() walks an unbounded number of list entries during the wake up
    and may wake an unbounded number of waiters.

For simplicity and performance reasons complete() should be usable on
PREEMPT_RT enabled kernels.

completions do not use custom wakeup callbacks and are usually single
waiter, except for a few corner cases.

Replace the wait queue in the completion with a simple wait queue (swait),
which uses a raw_spinlock_t for protecting the waiter list and therefore is
safe to use inside truly atomic regions on PREEMPT_RT.

There is no semantical or functional change:

  - completions use the exclusive wait mode which is what swait provides

  - complete() wakes one exclusive waiter

  - complete_all() wakes all waiters while holding the lock which protects
    the wait queue against newly incoming waiters. The conversion to swait
    preserves this behaviour.

complete_all() might cause unbound latencies with a large number of waiters
being woken at once, but most complete_all() usage sites are either in
testing or initialization code or have only a really small number of
concurrent waiters which for now does not cause a latency problem. Keep it
simple for now.

The fixup of the warning check in the USB gadget driver is just a straight
forward conversion of the lockless waiter check from one waitqueue type to
the other.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200321113242.317954042@linutronix.de
This commit is contained in:
Thomas Gleixner
2020-03-21 12:26:00 +01:00
committed by Peter Zijlstra
parent b3212fe2bc
commit a5c6234e10
3 changed files with 24 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
include/linux/completion.h
View File

@@ -9,7 +9,7 @@
* See kernel/sched/completion.c for details.
*/
#include <linux/wait.h>
#include <linux/swait.h>
/*
* struct completion - structure used to maintain state for a "completion"
@@ -25,7 +25,7 @@
*/
struct completion {
unsigned int done;
wait_queue_head_t wait;
struct swait_queue_head wait;
};
#define init_completion_map(x, m) __init_completion(x)
@@ -34,7 +34,7 @@ static inline void complete_acquire(struct completion *x) {}
static inline void complete_release(struct completion *x) {}
#define COMPLETION_INITIALIZER(work) \
{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
{ 0, __SWAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
#define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \
(*({ init_completion_map(&(work), &(map)); &(work); }))
@@ -85,7 +85,7 @@ static inline void complete_release(struct completion *x) {}
static inline void __init_completion(struct completion *x)
{
x->done = 0;
init_waitqueue_head(&x->wait);
init_swait_queue_head(&x->wait);
}
/**