Re: [PATCH v2 5/7] VFS: Avoid read-write deadlock in try_to_writeback_inodes_sb

[Kamal Mostafa <kamal@canonical.com>]

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On Fri, 2012-01-06 at 01:35 +0100, Jan Kara wrote:
> On Thu 08-12-11 10:04:35, Kamal Mostafa wrote:
> > From: Valerie Aurora <val@vaaconsulting.com>
> > 
> > Use trylock in try_to_writeback_inodes_sb to avoid read-write
> > deadlocks that could be triggered by freeze.

>   Christoph asked about what is the exact deadlock this patch tries to fix.
> I don't think you answered that. So can you elaborate please? Is it somehow
> connected with the fact that ext4 calls try_to_writeback_inodes_sb() with
> i_mutex held?
> 
> 								Honza

This was discussed in the thread
http://www.spinics.net/lists/linux-fsdevel/msg48754.html
Summarizing...

Jan>   What's exactly the deadlock trylock protects from here?
Jan>   Or is it just an optimization?

Val>   The trylock is an optimization Dave Chinner suggested.  The first
Val>   version I wrote acquired the lock and then checked vfs_is_frozen().

Dave>  It's not so much an optimisation, but the general case of avoiding
Dave>  read-write deadlocks such that freezing can trigger. I think remount
Dave>  can trigger the same deadlock as freezing, so the trylock avoids
Dave>  both deadlock cases rather than just working around the freeze
Dave>  problem....

 -Kamal


> > BugLink: https://bugs.launchpad.net/bugs/897421
> > Signed-off-by: Valerie Aurora <val@vaaconsulting.com>
> > Cc: Kamal Mostafa <kamal@canonical.com>
> > Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com>
> > [kamal@canonical.com: patch restructure]
> > Signed-off-by: Kamal Mostafa <kamal@canonical.com>
> > ---
> >  fs/fs-writeback.c |   13 ++++++++-----
> >  1 files changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index ea89b3f..3a80f1b 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb);
> >   * try_to_writeback_inodes_sb	-	start writeback if none underway
> >   * @sb: the superblock
> >   *
> > - * Invoke writeback_inodes_sb if no writeback is currently underway.
> > - * Returns 1 if writeback was started, 0 if not.
> > + * Invoke writeback_inodes_sb if no writeback is currently underway
> > + * and no one else holds the s_umount lock.  Returns 1 if writeback
> > + * was started, 0 if not.
> >   */
> >  int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
> >  {
> > @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb);
> >   * @sb: the superblock
> >   * @nr: the number of pages to write
> >   *
> > - * Invoke writeback_inodes_sb if no writeback is currently underway.
> > - * Returns 1 if writeback was started, 0 if not.
> > + * Invoke writeback_inodes_sb if no writeback is currently underway
> > + * and no one else holds the s_umount lock.  Returns 1 if writeback
> > + * was started, 0 if not.
> >   */
> >  int try_to_writeback_inodes_sb_nr(struct super_block *sb,
> >  				   unsigned long nr,
> >  				   enum wb_reason reason)
> >  {
> >  	if (!writeback_in_progress(sb->s_bdi)) {
> > -		down_read(&sb->s_umount);
> > +		if (!down_read_trylock(&sb->s_umount))
> > +		    return 0;
> >  		if (nr == 0)
> >  			writeback_inodes_sb(sb, reason);
> >  		else
> > -- 
> > 1.7.5.4
> > 


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