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Btrfs: fix race with freeze and free space inodes 

So we start our freeze, somebody comes in and does an fsync() on a file
where we have to commit a transaction for whatever reason, and we will
deadlock because the freeze is waiting on FS_FREEZE people to stop writing
to the file system, but the transaction is waiting for its free space inodes
to be written out, which are in turn waiting on sb_start_intwrite while
trying to write the file extents.  To fix this we'll just skip the
sb_start_intwrite() if we TRANS_JOIN_NOLOCK since we're being waited on by a
transaction commit so we're safe wrt to freeze and this will keep us from
deadlocking.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
This commit is contained in:
Josef Bacik 2012-09-14 11:22:38 -04:00 committed by Chris Mason
parent 6bbe3a9c80
commit 98114659e0
1 changed files with 11 additions and 2 deletions
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13
fs/btrfs/transaction.c
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@ -342,7 +342,15 @@ again:
if (!h)
return ERR_PTR(-ENOMEM);
if (!__sb_start_write(root->fs_info->sb, SB_FREEZE_FS, false)) {
/*
* If we are JOIN_NOLOCK we're already committing a transaction and
* waiting on this guy, so we don't need to do the sb_start_intwrite
* because we're already holding a ref. We need this because we could
* have raced in and did an fsync() on a file which can kick a commit
* and then we deadlock with somebody doing a freeze.
*/
if (type != TRANS_JOIN_NOLOCK &&
!__sb_start_write(root->fs_info->sb, SB_FREEZE_FS, false)) {
if (type == TRANS_JOIN_FREEZE)
return ERR_PTR(-EPERM);
sb_start_intwrite(root->fs_info->sb);
@ -601,7 +609,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
}
}
sb_end_intwrite(root->fs_info->sb);
if (lock)
sb_end_intwrite(root->fs_info->sb);
WARN_ON(cur_trans != info->running_transaction);
WARN_ON(atomic_read(&cur_trans->num_writers) < 1);