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Date:   Wed, 15 Jul 2020 18:01:44 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq

On Tue, Jul 14, 2020 at 07:37:14PM -0700, Eric Biggers wrote:
> On Wed, Jul 15, 2020 at 11:30:08AM +1000, Dave Chinner wrote:
> > On Sun, Jul 12, 2020 at 08:33:30PM -0700, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@...gle.com>
> > > 
> > > Fix the preliminary checks for ->s_dio_done_wq to use READ_ONCE(), since
> > > it's a data race, and technically the behavior is undefined without
> > > READ_ONCE().  Also, on one CPU architecture (Alpha), the data read
> > > dependency barrier included in READ_ONCE() is needed to guarantee that
> > > the pointed-to struct is seen as fully initialized.
> > > 
> > > Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> > > ---
> > >  fs/direct-io.c       | 8 +++-----
> > >  fs/internal.h        | 9 ++++++++-
> > >  fs/iomap/direct-io.c | 3 +--
> > >  3 files changed, 12 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/fs/direct-io.c b/fs/direct-io.c
> > > index 6d5370eac2a8..26221ae24156 100644
> > > --- a/fs/direct-io.c
> > > +++ b/fs/direct-io.c
> > > @@ -590,7 +590,7 @@ static inline int dio_bio_reap(struct dio *dio, struct dio_submit *sdio)
> > >   * filesystems that don't need it and also allows us to create the workqueue
> > >   * late enough so the we can include s_id in the name of the workqueue.
> > >   */
> > > -int sb_init_dio_done_wq(struct super_block *sb)
> > > +int __sb_init_dio_done_wq(struct super_block *sb)
> > >  {
> > >  	struct workqueue_struct *old;
> > >  	struct workqueue_struct *wq = alloc_workqueue("dio/%s",
> > > @@ -615,9 +615,7 @@ static int dio_set_defer_completion(struct dio *dio)
> > >  	if (dio->defer_completion)
> > >  		return 0;
> > >  	dio->defer_completion = true;
> > > -	if (!sb->s_dio_done_wq)
> > > -		return sb_init_dio_done_wq(sb);
> > > -	return 0;
> > > +	return sb_init_dio_done_wq(sb);
> > >  }
> > >  
> > >  /*
> > > @@ -1250,7 +1248,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
> > >  		retval = 0;
> > >  		if (iocb->ki_flags & IOCB_DSYNC)
> > >  			retval = dio_set_defer_completion(dio);
> > > -		else if (!dio->inode->i_sb->s_dio_done_wq) {
> > > +		else {
> > >  			/*
> > >  			 * In case of AIO write racing with buffered read we
> > >  			 * need to defer completion. We can't decide this now,
> > > diff --git a/fs/internal.h b/fs/internal.h
> > > index 9b863a7bd708..6736c9eee978 100644
> > > --- a/fs/internal.h
> > > +++ b/fs/internal.h
> > > @@ -178,7 +178,14 @@ extern void mnt_pin_kill(struct mount *m);
> > >  extern const struct dentry_operations ns_dentry_operations;
> > >  
> > >  /* direct-io.c: */
> > > -int sb_init_dio_done_wq(struct super_block *sb);
> > > +int __sb_init_dio_done_wq(struct super_block *sb);
> > > +static inline int sb_init_dio_done_wq(struct super_block *sb)
> > > +{
> > > +	/* pairs with cmpxchg() in __sb_init_dio_done_wq() */
> > > +	if (likely(READ_ONCE(sb->s_dio_done_wq)))
> > > +		return 0;
> > > +	return __sb_init_dio_done_wq(sb);
> > > +}
> > 
> > Ummm, why don't you just add this check in sb_init_dio_done_wq(). I
> > don't see any need for adding another level of function call
> > abstraction in the source code?
> 
> This keeps the fast path doing no function calls and one fewer branch, as it was
> before.  People care a lot about minimizing direct I/O overhead, so it seems
> desirable to keep this simple optimization.  Would you rather it be removed?

No.

What I'm trying to say is that I'd prefer fast path checks don't get
hidden away in a static inline function wrappers that require the
reader to go look up code in a different file to understand that
code in yet another different file is conditionally executed.

Going from obvious, easy to read fast path code to spreading the
fast path logic over functions in 3 different files is not an
improvement in the code - it is how we turn good code into an
unmaintainable mess...

> > Also, you need to explain the reason for the READ_ONCE() existing
> > rather than just saying "it pairs with <some other operation>".
> > Knowing what operation it pairs with doesn't explain why the pairing
> > is necessary in the first place, and that leads to nobody reading
> > the code being able to understand what this is protecting against.
> > 
> 
> How about this?
> 
> 	/*
> 	 * Nothing to do if ->s_dio_done_wq is already set.  But since another
> 	 * process may set it concurrently, we need to use READ_ONCE() rather
> 	 * than a plain read to avoid a data race (undefined behavior) and to
> 	 * ensure we observe the pointed-to struct to be fully initialized.
> 	 */
> 	if (likely(READ_ONCE(sb->s_dio_done_wq)))
> 		return 0;

You still need to document what it pairs with, as "data race" doesn't
describe the actual dependency we are synchronising against is.

AFAICT from your description, the data race is not on
sb->s_dio_done_wq itself, but on seeing the contents of the
structure being pointed to incorrectly. i.e. we need to ensure that
writes done before the cmpxchg are ordered correctly against
reads done after the pointer can be seen here.

If so, can't we just treat this as a normal
store-release/load-acquire ordering pattern and hence use more
relaxed memory barriers instead of have to patch up what we have now
to specifically make ancient platforms that nobody actually uses
with weird and unusual memory models work correctly?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com

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