Re: [PATCH] btrfs: zoned: move log tree node allocation out of log_root_tree->log_mutex
From: Filipe Manana <hidden>
Date: 2021-03-24 14:32:50
On Wed, Mar 24, 2021 at 2:26 PM Naohiro Aota [off-list ref] wrote:
Commit 6e37d2459941 ("btrfs: zoned: fix deadlock on log sync") pointed out
a deadlock warning and removed
mutex_{lock,unlock}(&fs_info->tree_root->log_mutex). While it looks like it
always cause a deadlock, we didn't see actual deadlock in fstests runs. The
reason is log_root_tree->log_mutex != fs_info->tree_root->log_mutex, not
taking the same lock. So, the warning was actually a false-positive.
Since btrfs_alloc_log_tree_node() is protected only by
fs_info->tree_root->log_mutex, we can (and should) move the code out of the
lock scope of log_root_tree->log_mutex and silence the warning.
Cc: Filipe Manana <redacted>
Cc: Johannes Thumshirn <redacted>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>Reviewed-by: Filipe Manana <redacted> Looks good, thanks.
quoted hunk ↗ jump to hunk
--- fs/btrfs/tree-log.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index 92a368627791..72c4b66ed516 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c@@ -3165,20 +3165,22 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans, */ mutex_unlock(&root->log_mutex); - btrfs_init_log_ctx(&root_log_ctx, NULL); - - mutex_lock(&log_root_tree->log_mutex); - if (btrfs_is_zoned(fs_info)) { + mutex_lock(&fs_info->tree_root->log_mutex); if (!log_root_tree->node) { ret = btrfs_alloc_log_tree_node(trans, log_root_tree); if (ret) { - mutex_unlock(&log_root_tree->log_mutex); + mutex_unlock(&fs_info->tree_log_mutex); goto out; } } + mutex_unlock(&fs_info->tree_root->log_mutex); } + btrfs_init_log_ctx(&root_log_ctx, NULL); + + mutex_lock(&log_root_tree->log_mutex); + index2 = log_root_tree->log_transid % 2; list_add_tail(&root_log_ctx.list, &log_root_tree->log_ctxs[index2]); root_log_ctx.log_transid = log_root_tree->log_transid; --2.31.0
-- Filipe David Manana, “Whether you think you can, or you think you can't — you're right.”