[PATCH 3.16 175/306] btrfs: fix races on root_log_ctx lists

Ben Hutchings
2017-02-15T23:24:09Z

From: Chris Mason <clm@fb.com>

commit 570dd45042a7c8a7aba1ee029c5dd0f5ccf41b9b upstream.

btrfs_remove_all_log_ctxs takes a shortcut where it avoids walking the
list because it knows all of the waiters are patiently waiting for the
commit to finish.

But, there's a small race where btrfs_sync_log can remove itself from
the list if it finds a log commit is already done. Also, it uses
list_del_init() to remove itself from the list, but there's no way to
know if btrfs_remove_all_log_ctxs has already run, so we don't know for
sure if it is safe to call list_del_init().

This gets rid of all the shortcuts for btrfs_remove_all_log_ctxs(), and
just calls it with the proper locking.

This is part two of the corruption fixed by cbd60aa7cd1. I should have
done this in the first place, but convinced myself the optimizations were
safe. A 12 hour run of dbench 2048 will eventually trigger a list debug
WARN_ON for the list_del_init() in btrfs_sync_log().

Fixes: d1433debe7f4346cf9fc0dafc71c3137d2a97bc4
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/btrfs/tree-log.c | 20 ++++++--------------
1 file changed, 6 insertions(+), 14 deletions(-)

--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2449,14 +2449,12 @@ static inline void btrfs_remove_all_log_
int index, int error)
{
struct btrfs_log_ctx *ctx;
+ struct btrfs_log_ctx *safe;

- if (!error) {
- INIT_LIST_HEAD(&root->log_ctxs[index]);
- return;
- }
-
- list_for_each_entry(ctx, &root->log_ctxs[index], list)
+ list_for_each_entry_safe(ctx, safe, &root->log_ctxs[index], list) {
+ list_del_init(&ctx->list);
ctx->log_ret = error;
+ }

INIT_LIST_HEAD(&root->log_ctxs[index]);
}
@@ -2686,13 +2684,9 @@ int btrfs_sync_log(struct btrfs_trans_ha
mutex_unlock(&root->log_mutex);

out_wake_log_root:
- /*
- * We needn't get log_mutex here because we are sure all
- * the other tasks are blocked.
- */
+ mutex_lock(&log_root_tree->log_mutex);
btrfs_remove_all_log_ctxs(log_root_tree, index2, ret);

- mutex_lock(&log_root_tree->log_mutex);
log_root_tree->log_transid_committed++;
atomic_set(&log_root_tree->log_commit[index2], 0);
mutex_unlock(&log_root_tree->log_mutex);
@@ -2700,10 +2694,8 @@ out_wake_log_root:
if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
wake_up(&log_root_tree->log_commit_wait[index2]);
out:
- /* See above. */
- btrfs_remove_all_log_ctxs(root, index1, ret);
-
mutex_lock(&root->log_mutex);
+ btrfs_remove_all_log_ctxs(root, index1, ret);
root->log_transid_committed++;
atomic_set(&root->log_commit[index1], 0);
mutex_unlock(&root->log_mutex);