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Date: Fri, 17 Jul 2015 11:27:42 +1000
From: Dave Chinner <david@fromorbit.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Dave Hansen <dave.hansen@linux.intel.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 0/4] change sb_writers to use percpu_rw_semaphore
Message-ID: <20150717012742.GV3902@dastard>
References: <20150713212536.GA13855@redhat.com>
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 <55A57C50.1080406@linux.intel.com>
 <20150714212249.GA18441@redhat.com>
 <55A581F9.3090507@linux.intel.com>
 <20150715064705.GA22609@quack.suse.cz>
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On Thu, Jul 16, 2015 at 07:32:56PM +0200, Oleg Nesterov wrote:
> 
> 	#ifdef CONFIG_LOCKDEP
> 		/*
> 		 * We want lockdep to tell us about possible deadlocks with freezing but
> 		 * it's it bit tricky to properly instrument it. Getting a freeze protection
> 		 * works as getting a read lock but there are subtle problems. XFS for example
> 		 * gets freeze protection on internal level twice in some cases, which is OK

Sorry, I've missed something here - where is XFS nesting
sb_start_intwrite() calls?  XFS only has a call to
sb_start_intwrite() in xfs_trans_alloc() and an open coded
equivalent in xfs_iomap_write_unwritten(). However, we cannot create
nested transaction contexts as doing so is *guaranteed to deadlock
the journal*. (e.g. why we use PF_FSTRANS to trigger GFP_NOFS
allocation as it prevents direct memory reclaim from causing nested
transactions.) Hence if we taking multiple FREEZE_FS level
references at a time then there is a bug that needs fixing....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com