Re: [PATCH 1/2 v2] wait: add wait_event_lock_irq() interface

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 22 Nov 2012 09:18:34 +0100
Lukas Czerner <lczerner@redhat.com> wrote:

> New wait_event{_interruptible}_lock_irq{_cmd} macros added. This commit
> moves the private wait_event_lock_irq() macro from MD to regular wait
> includes, introduces new macro wait_event_lock_irq_cmd() instead of using
> the old method with omitting cmd parameter which is ugly and makes a use
> of new macros in the MD. It also introduces the _interruptible_ variant.
> 
> The use of new interface is when one have a special lock to protect data
> structures used in the condition, or one also needs to invoke "cmd"
> before putting it to sleep. "cmd" is always invoked before checking the
> "condition" so in case it will change the condition outcome we would not
> have to sleep unnecessarily.
> 
> All new macros are expected to be called with the lock taken. The lock
> is released before sleep and is reacquired afterwards. We will leave the
> macro with the lock held.
> 
> Note to DM: IMO this should also fix theoretical race on waitqueue while
> using simultaneously wait_event_lock_irq() and wait_event() because of
> lack of locking around current state setting and wait queue removal.
> 
> ...
>
> +#define __wait_event_lock_irq(wq, condition, lock, cmd)			\
> +do {									\
> +	DEFINE_WAIT(__wait);						\
> +									\
> +	cmd;								\
> +	for (;;) {							\
> +		prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);	\
> +		if (condition)						\
> +			break;						\
> +		spin_unlock_irq(&lock);					\
> +		schedule();						\
> +		cmd;							\
> +		spin_lock_irq(&lock);					\
> +	}								\
> +	finish_wait(&wq, &__wait);					\
> +} while (0)
>
> ...
>
> +#define __wait_event_interruptible_lock_irq(wq, condition,		\
> +					    lock, ret, cmd)		\
> +do {									\
> +	DEFINE_WAIT(__wait);						\
> +									\
> +	cmd;								\
> +	for (;;) {							\
> +		prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE);	\
> +		if (condition)						\
> +			break;						\
> +		if (signal_pending(current)) {				\
> +			ret = -ERESTARTSYS;				\
> +			break;						\
> +		}							\
> +		spin_unlock_irq(&lock);					\
> +		schedule();						\
> +		cmd;							\
> +		spin_lock_irq(&lock);					\
> +	}								\
> +	finish_wait(&wq, &__wait);					\
> +} while (0)

These could be combined - use

	if (flags == TASK_INTERRUPTIBLE && signal_pending(current)) {	\
		ret = -ERESTARTSYS;					\
		break;							\
	}								\

and the compiler will fully remove that code for the
__wait_event_lock_irq() case.

But that's all a pretty small gain, IMO.