[GIT PULL] vfs: fs freeze fix for 5.10-rc4

[GIT PULL] vfs: fs freeze fix for 5.10-rc4

[Darrick J. Wong]

Hi Linus,

Please pull this branch containing a single vfs fix for 5.10-rc4.  A
very long time ago, a hack was added to the vfs fs freeze protection
code to work around lockdep complaints about XFS, which would try to run
a transaction (which requires intwrite protection) to finalize an xfs
freeze (by which time the vfs had already taken intwrite).

Fast forward a few years, and XFS fixed the recursive intwrite problem
on its own, and the hack became unnecessary.  Fast forward almost a
decade, and latent bugs in the code converting this hack from freeze
flags to freeze locks combine with lockdep bugs to make this reproduce
frequently enough to notice page faults racing with freeze.

Since the hack is unnecessary and causes thread race errors, just get
rid of it completely.  Pushing this kind of vfs change midway through a
cycle makes me nervous, but a large enough number of the usual
VFS/ext4/XFS/btrfs suspects have said this looks good and solves a real
problem vector, so I'm sending this for your consideration instead of
holding off until 5.11.

The branch merges cleanly with upstream as of a few minutes ago, so
please let me know if anything strange happens.

--D

The following changes since commit 3650b228f83adda7e5ee532e2b90429c03f7b9ec:

  Linux 5.10-rc1 (2020-10-25 15:14:11 -0700)

are available in the Git repository at:

  git://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git tags/vfs-5.10-fixes-1

for you to fetch changes up to 22843291efc986ce7722610073fcf85a39b4cb13:

  vfs: remove lockdep bogosity in __sb_start_write (2020-11-10 16:49:29 -0800)

----------------------------------------------------------------
VFS fixes for 5.10-rc4:
- Finally remove the "convert to trylock" weirdness in the fs freezer
  code.  It was necessary 10 years ago to deal with nested transactions
  in XFS, but we've long since removed that; and now this is causing
  subtle race conditions when lockdep goes offline and sb_start_* aren't
  prepared to retry a trylock failure.

----------------------------------------------------------------
Darrick J. Wong (1):
      vfs: remove lockdep bogosity in __sb_start_write

 fs/super.c | 33 ++++-----------------------------
 1 file changed, 4 insertions(+), 29 deletions(-)

Re: [GIT PULL] vfs: fs freeze fix for 5.10-rc4

[Linus Torvalds]

On Fri, Nov 13, 2020 at 3:38 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> Since the hack is unnecessary and causes thread race errors, just get
> rid of it completely.  Pushing this kind of vfs change midway through a
> cycle makes me nervous, but a large enough number of the usual
> VFS/ext4/XFS/btrfs suspects have said this looks good and solves a real
> problem vector, so I'm sending this for your consideration instead of
> holding off until 5.11.

Not a fan of the timing, but you make a good argument, and I love
seeing code removal. So I took it.

And once I took the real code change, the two cleanups looked like the
least of the problem, so I took them too.

I ended up doing it all just as a single pull, since it seemed
pointless to make history more complicated just to separate out the
cleanups in a separate pull.

Now I really hope this won't cause any problems, but it certainly
_looks_ harmless.

          Linus

Re: [GIT PULL] vfs: fs freeze fix for 5.10-rc4

[Darrick J. Wong]

On Fri, Nov 13, 2020 at 04:13:37PM -0800, Linus Torvalds wrote:
> On Fri, Nov 13, 2020 at 3:38 PM Darrick J. Wong <djwong@kernel.org> wrote:
> >
> > Since the hack is unnecessary and causes thread race errors, just get
> > rid of it completely.  Pushing this kind of vfs change midway through a
> > cycle makes me nervous, but a large enough number of the usual
> > VFS/ext4/XFS/btrfs suspects have said this looks good and solves a real
> > problem vector, so I'm sending this for your consideration instead of
> > holding off until 5.11.
> 
> Not a fan of the timing, but you make a good argument, and I love
> seeing code removal. So I took it.

Thanks!  Admittedly this is super late because it didn't occur to me
until after -rc1 that the periodic hangs I saw in for-next were related
to lockdep being broken and weren't some other weird vfs/xfs breakage;
and then I wanted to spin this patch through my internal testing systems
for a week to convince myself that changing the freeze locking code
wasn't totally nuts. :)

--D

> And once I took the real code change, the two cleanups looked like the
> least of the problem, so I took them too.
> 
> I ended up doing it all just as a single pull, since it seemed
> pointless to make history more complicated just to separate out the
> cleanups in a separate pull.
> 
> Now I really hope this won't cause any problems, but it certainly
> _looks_ harmless.
> 
>           Linus
> 

Re: [GIT PULL] vfs: fs freeze fix for 5.10-rc4

[pr-tracker-bot]

The pull request you sent on Fri, 13 Nov 2020 15:38:47 -0800:

> git://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git tags/vfs-5.10-fixes-1

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/8a3c84b649b033024d2349f96234b26cbd6083a6

Thank you!

-- 
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