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

[Mikulas Patocka]

On Wed, 11 Jan 2012, Kamal Mostafa wrote:

> 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@xxxxxxxxxxxxxxxxx>
> > > 
> > > 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

As I wrote in 
https://www.redhat.com/archives/dm-devel/2011-November/msg00151.html , 
down_read_trylock doesn't fix the freeze deadlock. Think of this sequence:

Process 1 (freezing)
down_write(&sb->s_umount);
set the filesystem to frozen state
up_write(&sb->s_umount);

Process 2 (executing the code from the patch)
down_read_trylock(&sb->s_umount); - succeeds, because s_umount is not held
writeback_inodes_sb(sb, reason); - waits, because the filesystem is frozen

Process 1 (unfreezing)
down_write(&sb->s_umount); - deadlock (process 1 is waiting for process 2 
to drop the lock; process 2 is waiting for process 1 to unfreeze).

See the patch at 
https://www.redhat.com/archives/dm-devel/2011-November/msg00151.html , it 
has a different approach and it avoids the mentined freeze deadlock.

Mikulas

> > > BugLink: https://bugs.launchpad.net/bugs/897421
> > > Signed-off-by: Valerie Aurora <val@xxxxxxxxxxxxxxxxx>
> > > Cc: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
> > > Tested-by: Peter M. Petrakis <peter.petrakis@xxxxxxxxxxxxx>
> > > [kamal@xxxxxxxxxxxxx: patch restructure]
> > > Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
> > > ---
> > >  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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