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Btrfs: fix race between start dirty bg cache writeout and bg deletion 

While running xfstests I ran into the following:

[20892.242791] ------------[ cut here ]------------
[20892.243776] WARNING: CPU: 0 PID: 13299 at fs/btrfs/super.c:260 __btrfs_abort_transaction+0x52/0x114 [btrfs]()
[20892.245874] BTRFS: Transaction aborted (error -2)
[20892.247329] Modules linked in: btrfs dm_snapshot dm_bufio dm_flakey dm_mod crc32c_generic xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache sunrpc loop fuse$
[20892.258488] CPU: 0 PID: 13299 Comm: fsstress Tainted: G        W       4.0.0-rc5-btrfs-next-9+ #2
[20892.262011] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
[20892.264738]  0000000000000009 ffff880427f8bc18 ffffffff8142fa46 ffffffff8108b6a2
[20892.266244]  ffff880427f8bc68 ffff880427f8bc58 ffffffff81045ea5 ffff880427f8bc48
[20892.267761]  ffffffffa0509a6d 00000000fffffffe ffff8803545d6f40 ffffffffa05a15a0
[20892.269378] Call Trace:
[20892.269915]  [<ffffffff8142fa46>] dump_stack+0x4f/0x7b
[20892.271097]  [<ffffffff8108b6a2>] ? console_unlock+0x361/0x3ad
[20892.272173]  [<ffffffff81045ea5>] warn_slowpath_common+0xa1/0xbb
[20892.273386]  [<ffffffffa0509a6d>] ? __btrfs_abort_transaction+0x52/0x114 [btrfs]
[20892.274857]  [<ffffffff81045f05>] warn_slowpath_fmt+0x46/0x48
[20892.275851]  [<ffffffffa0509a6d>] __btrfs_abort_transaction+0x52/0x114 [btrfs]
[20892.277341]  [<ffffffffa0515e10>] write_one_cache_group+0x68/0xaf [btrfs]
[20892.278628]  [<ffffffffa052088a>] btrfs_start_dirty_block_groups+0x18d/0x29b [btrfs]
[20892.280191]  [<ffffffffa052f077>] btrfs_commit_transaction+0x130/0x9c9 [btrfs]
[20892.281781]  [<ffffffff8107d33d>] ? trace_hardirqs_on+0xd/0xf
[20892.282873]  [<ffffffffa054163b>] btrfs_sync_file+0x313/0x387 [btrfs]
[20892.284111]  [<ffffffff8117acad>] vfs_fsync_range+0x95/0xa4
[20892.285203]  [<ffffffff810e603f>] ? time_hardirqs_on+0x15/0x28
[20892.286290]  [<ffffffff8123960b>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[20892.287469]  [<ffffffff8117acd8>] vfs_fsync+0x1c/0x1e
[20892.288412]  [<ffffffff8117ae54>] do_fsync+0x34/0x4e
[20892.289348]  [<ffffffff8117b07c>] SyS_fsync+0x10/0x14
[20892.290255]  [<ffffffff81435b32>] system_call_fastpath+0x12/0x17
[20892.291316] ---[ end trace 597f77e664245373 ]---
[20892.293955] BTRFS: error (device sdg) in write_one_cache_group:3184: errno=-2 No such entry
[20892.297390] BTRFS info (device sdg): forced readonly

This happens because in btrfs_start_dirty_block_groups() we splice the
transaction's list of dirty block groups into a local list and then we
keep extracting the first element of the list without holding the
cache_write_mutex mutex. This means that before we acquire that mutex
the first block group on the list might be removed by a conurrent task
running btrfs_remove_block_group(). So make sure we extract the first
element (and test the list emptyness) while holding that mutex.

Fixes: 1bbc621ef2 ("Btrfs: allow block group cache writeout
                      outside critical section in commit")

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
This commit is contained in:
Filipe Manana 2015-04-25 18:29:16 +01:00 committed by Chris Mason
parent a3bdccc4e6
commit b58d1a9ef9
1 changed files with 27 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
fs/btrfs/extent-tree.c
View File

@ -3408,17 +3408,14 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
int loops = 0;
spin_lock(&cur_trans->dirty_bgs_lock);
if (!list_empty(&cur_trans->dirty_bgs)) {
list_splice_init(&cur_trans->dirty_bgs, &dirty);
if (list_empty(&cur_trans->dirty_bgs)) {
spin_unlock(&cur_trans->dirty_bgs_lock);
return 0;
}
list_splice_init(&cur_trans->dirty_bgs, &dirty);
spin_unlock(&cur_trans->dirty_bgs_lock);
again:
if (list_empty(&dirty)) {
btrfs_free_path(path);
return 0;
}
/*
* make sure all the block groups on our dirty list actually
* exist
@ -3431,18 +3428,16 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
return -ENOMEM;
}
/*
* cache_write_mutex is here only to save us from balance or automatic
* removal of empty block groups deleting this block group while we are
* writing out the cache
*/
mutex_lock(&trans->transaction->cache_write_mutex);
while (!list_empty(&dirty)) {
cache = list_first_entry(&dirty,
struct btrfs_block_group_cache,
dirty_list);
/*
* cache_write_mutex is here only to save us from balance
* deleting this block group while we are writing out the
* cache
*/
mutex_lock(&trans->transaction->cache_write_mutex);
/*
* this can happen if something re-dirties a block
* group that is already under IO. Just wait for it to
@ -3495,7 +3490,6 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
}
if (!ret)
ret = write_one_cache_group(trans, root, path, cache);
mutex_unlock(&trans->transaction->cache_write_mutex);
/* if its not on the io list, we need to put the block group */
if (should_put)
@ -3503,7 +3497,16 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
if (ret)
break;
/*
* Avoid blocking other tasks for too long. It might even save
* us from writing caches for block groups that are going to be
* removed.
*/
mutex_unlock(&trans->transaction->cache_write_mutex);
mutex_lock(&trans->transaction->cache_write_mutex);
}
mutex_unlock(&trans->transaction->cache_write_mutex);
/*
* go through delayed refs for all the stuff we've just kicked off
@ -3514,8 +3517,15 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
loops++;
spin_lock(&cur_trans->dirty_bgs_lock);
list_splice_init(&cur_trans->dirty_bgs, &dirty);
/*
* dirty_bgs_lock protects us from concurrent block group
* deletes too (not just cache_write_mutex).
*/
if (!list_empty(&dirty)) {
spin_unlock(&cur_trans->dirty_bgs_lock);
goto again;
}
spin_unlock(&cur_trans->dirty_bgs_lock);
goto again;
}
btrfs_free_path(path);