linux-kernel - Re: [PATCH pm-for-3.2 1/2] freezer: reimplement wait_event_freezekillable using freezer_do_not_count/freezer

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Date:	Fri, 4 Nov 2011 01:13:07 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	Jeff Layton <jlayton@...hat.com>
Subject: Re: [PATCH pm-for-3.2 1/2] freezer: reimplement wait_event_freezekillable using freezer_do_not_count/freezer_count()

On Friday, November 04, 2011, Tejun Heo wrote:
> From: Oleg Nesterov <oleg@...hat.com>
> 
> Commit 27920651fe "PM / Freezer: Make fake_signal_wake_up() wake
> TASK_KILLABLE tasks too" updated fake_signal_wake_up() used by freezer
> to wake up KILLABLE tasks.  Sending unsolicited wakeups to tasks in
> killable sleep is dangerous as there are code paths which depend on
> tasks not waking up spuriously from KILLABLE sleep.
> 
> For example. sys_read() or page can sleep in TASK_KILLABLE assuming
> that wait/down/whatever _killable can only fail if we can not return
> to the usermode.  TASK_TRACED is another obvious example.
> 
> The offending commit was to resolve freezer hang during system PM
> operations caused by KILLABLE sleeps in network filesystems.
> wait_event_freezekillable(), which depends on the spurious KILLABLE
> wakeup, was added by f06ac72e92 "cifs, freezer: add
> wait_event_freezekillable and have cifs use it" to be used to
> implement killable & freezable sleeps in network filesystems.
> 
> To prepare for reverting of 27920651fe, this patch reimplements
> wait_event_freezekillable() using freezer_do_not_count/freezer_count()
> so that it doesn't depend on the spurious KILLABLE wakeup.  This isn't
> very nice but should do for now.
> 
> tj: Refreshed patch to apply to linus/master and updated commit
>     description on Rafael's request.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Jeff Layton <jlayton@...hat.com>
> LKML-Reference: <20111102175327.GA4446@...hat.com>

Applied to linux-pm/linux-next.

Thanks,
Rafael


> ---
>  include/linux/freezer.h |   11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> Index: work/include/linux/freezer.h
> ===================================================================
> --- work.orig/include/linux/freezer.h
> +++ work/include/linux/freezer.h
> @@ -143,14 +143,9 @@ static inline void set_freezable_with_si
>  #define wait_event_freezekillable(wq, condition)			\
>  ({									\
>  	int __retval;							\
> -	do {								\
> -		__retval = wait_event_killable(wq,			\
> -				(condition) || freezing(current));	\
> -		if (__retval && !freezing(current))			\
> -			break;						\
> -		else if (!(condition))					\
> -			__retval = -ERESTARTSYS;			\
> -	} while (try_to_freeze());					\
> +	freezer_do_not_count();						\
> +	__retval = wait_event_killable(wq, (condition));		\
> +	freezer_count();						\
>  	__retval;							\
>  })
>  
> 
> 

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