linux-kernel - Re: [PATCH v3 02/16] freezer: add unsafe versions of freezable helpers for CIFS

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Date:	Tue, 7 May 2013 06:07:30 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-nfs@...r.kernel.org,
	linux-pm@...r.kernel.org, netdev@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Len Brown <len.brown@...el.com>
Subject: Re: [PATCH v3 02/16] freezer: add unsafe versions of freezable
 helpers for CIFS

On Mon,  6 May 2013 16:50:07 -0700
Colin Cross <ccross@...roid.com> wrote:

> CIFS calls wait_event_freezekillable_unsafe with a VFS lock held,
> which is unsafe and will cause lockdep warnings when 6aa9707
> "lockdep: check that no locks held at freeze time" is reapplied
> (it was reverted in dbf520a).  CIFS shouldn't be doing this, but
> it has long-running syscalls that must hold a lock but also
> shouldn't block suspend.  Until CIFS freeze handling is rewritten
> to use a signal to exit out of the critical section, add a new
> wait_event_freezekillable_unsafe helper that will not run the
> lockdep test when 6aa9707 is reapplied, and call it from CIFS.
> 
> In practice the likley result of holding the lock while freezing
> is that a second task blocked on the lock will never freeze,
> aborting suspend, but it is possible to manufacture a case using
> the cgroup freezer, the lock, and the suspend freezer to create
> a deadlock.  Silencing the lockdep warning here will allow
> problems to be found in other drivers that may have a more
> serious deadlock risk, and prevent new problems from being added.
> 
> Signed-off-by: Colin Cross <ccross@...roid.com>
> ---
>  include/linux/freezer.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/freezer.h b/include/linux/freezer.h
> index 5b31e21c..d3c038e 100644
> --- a/include/linux/freezer.h
> +++ b/include/linux/freezer.h
> @@ -212,6 +212,16 @@ static inline bool freezer_should_skip(struct task_struct *p)
>  	__retval;							\
>  })
>  
> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
> +#define wait_event_freezekillable_unsafe(wq, condition)			\
> +({									\
> +	int __retval;							\
> +	freezer_do_not_count();						\
> +	__retval = wait_event_killable(wq, (condition));		\
> +	freezer_count_unsafe();						\
> +	__retval;							\
> +})
> +
>  #define wait_event_freezable(wq, condition)				\
>  ({									\
>  	int __retval;							\
> @@ -277,6 +287,9 @@ static inline void set_freezable(void) {}
>  #define wait_event_freezekillable(wq, condition)		\
>  		wait_event_killable(wq, condition)
>  
> +#define wait_event_freezekillable_unsafe(wq, condition)			\
> +		wait_event_killable(wq, condition)
> +
>  #endif /* !CONFIG_FREEZER */
>  
>  #endif	/* FREEZER_H_INCLUDED */

I think you also need to convert wait_for_response in the cifs code to
use this helper. While it's a pretty straightforward change, you should
probably cc linux-cifs@...r.kernel.org as well.

-- 
Jeff Layton <jlayton@...hat.com>
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