Re: [PATCH] btrfs: Fix locking during DIO read

[David Sterba <dsterba@suse.cz>]

On Thu, Feb 22, 2018 at 12:24:40PM -0700, Liu Bo wrote:
> > > > Not even that far, isize is truncated before calling inode_dio_wait()
> > > > and a memory barrier is set to ensure the correct order, so dio read
> > > > would simply return if it's reading past isize.
> > > 
> > > Please, describe concretely which meory barriers pairs with chich in
> > > order to ensure proper visibility. Because I'm of the opinion there are
> > > insufficient memoru barriers to ensure setting READDIO_LOCK is properly
> > > visible in btrfs_direct_IO. Since the latter has no barriers whatsoever.
> > 
> > smp_mb() is supposed to be paired, so there is one missing, I agree.
> >
> 
> So the missing smp_mb() was there (commit
> 2e60a51e62185cce48758e596ae7cb2da673b58f), but was removed in some
> cleanup I guess.

Not a cleanup, and not a single commit:

38851cc19adbfa1def2b47106d8050a80e0a3673
Btrfs: serialize unlocked dio reads with truncate
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>

at this time the i_dio_counter was still used directly and the barrier
was moved before the if-else block, technically still in the right
before the test_bit.

dc59215d4f42084ee13654bafe3e5130b146aeb7
btrfs: remove unnecessary memory barrier in btrfs_direct_IO
Signed-off-by: Nikolay Borisov <nborisov@suse.com>

... simply removes the barrier.