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

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Message-ID: <20130507122803.GA2585@elf.ucw.cz>
Date:	Tue, 7 May 2013 14:28:04 +0200
From:	Pavel Machek <pavel@....cz>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, "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 2013-05-06 16:50:07, Colin Cross 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>

Acked-by: Pavel Machek <pavel@....cz>

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