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btrfs: remove unnecessary memory barrier in btrfs_direct_IO

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Commit 38851cc19a ("Btrfs: implement unlocked dio write") implemented
unlocked dio write, allowing multiple dio writers to write to
non-overlapping, and non-eof-extending regions. In doing so it also
introduced a broken memory barrier. It is broken due to 2 things:

1. Memory barriers _MUST_ always be paired, this is clearly not the case
   here

2. Checkpatch actually produces a warning if a memory barrier is
   introduced that doesn't have a comment explaining how it's being
   paired.

Specifically for inode::i_dio_count that's wrapped inside
inode_dio_begin, there is no explicit barrier semantics attached, so
removing is fine as the atomic is used in common the waiter/wakeup
pattern.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ enhance changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov 2017-08-01 18:25:56 +03:00 committed by David Sterba
parent b5d9071c4f
commit dc59215d4f
1 changed files with 0 additions and 1 deletions
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fs/btrfs/inode.c
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@ -8815,7 +8815,6 @@ static ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
return 0;
inode_dio_begin(inode);
smp_mb__after_atomic();
/*
* The generic stuff only does filemap_write_and_wait_range, which