linux-kernel - Re: [PATCH driver-core-linus] sysfs: bail early from sysfs_bin

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Date:	Wed, 4 Dec 2013 09:13:45 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Dave Jones <davej@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	gregkh@...uxfoundation.org
Subject: Re: [PATCH driver-core-linus] sysfs: bail early from
 sysfs_bin_mmap() to avoid spurious lockdep warning

On Wed, Dec 04, 2013 at 09:06:39AM -0500, Tejun Heo wrote:
> 027a485d12e0 ("sysfs: use a separate locking class for open files
> depending on mmap") assigned different lockdep key to
> sysfs_open_file->mutex depending on whether the file implements mmap
> or not in an attempt to avoid spurious lockdep warning caused by
> merging of regular and bin file paths.
> 
> While this restored some of the original behavior of using different
> locks (at least lockdep is concerned) for the different clases of
> files.  The restoration wasn't full because now the lockdep key
> assignment depends on whether the file has mmap or not instead of
> whether it's a regular file or not.
> 
> This means that bin files which don't implement mmap will get assigned
> the same lockdep class as regular files.  This is problematic because
> file_operations for bin files still implements the mmap file operation
> and checking whether the sysfs file actually implements mmap happens
> in the file operation after grabbing @sysfs_open_file->mutex.  We
> still end up adding locking dependency from mmap locking to
> sysfs_open_file->mutex to the regular file mutex which triggers
> spurious circular locking warning.
> 
> This can be fixed by either giving sysfs_open_file->mutex different
> lockdep keys depending on whether the file is regular or bin instead
> of whether mmap exists or not, or avoiding grabbing
> sysfs_open_file->mutex from sysfs_bin_mmap() if mmap is not actually
> implemented.  While the former is simpler for driver-core-linus,
> driver-core-next already has SYSFS_FLAG_HAS_MMAP in place to implement
> the latter and doesn't have inherent distinction between regular and
> bin files.  This patch implements the latter so that the fix is more
> conducive to driver-core-next.
> 
> Because anything beyond sysfs_open_file->sd can't be dereferenced
> without locking the open file, cache whether mmap is implemented or
> not in sysfs_open_file->sd->s_flags and update sysfs_bin_mmap() test
> the flag and bail without grabbing the mutex if not implemented.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Dave Jones <davej@...hat.com>
> Tested-by: Dave Jones <davej@...hat.com>
> Link: http://lkml.kernel.org/g/20131203184324.GA11320@redhat.com

Please forget about this.  This is actually a lot harder to translate
to kernfs.  I'll prepare another patch.  Sorry about the noise.

Thanks.

-- 
tejun
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