From 99d16cbcaf694c803a1b6bf7e851694ffe1d255d Mon Sep 17 00:00:00 2001 From: Sage Weil Date: Fri, 29 Oct 2010 15:37:34 -0400 Subject: [PATCH] Btrfs: fix deadlock in btrfs_commit_transaction We calculate timeout (either 1 or MAX_SCHEDULE_TIMEOUT) based on whether num_writers > 1 or should_grow at the top of the loop. Then, much much later, we wait for that timeout if either num_writers or should_grow is true. However, it's possible for a racing process (calling btrfs_end_transaction()) to decrement num_writers such that we wait forever instead of for 1. Fix this by deciding how long to wait when we wait. Include a smp_mb() before checking if the waitqueue is active to ensure the num_writers is visible. Signed-off-by: Sage Weil Signed-off-by: Chris Mason --- fs/btrfs/transaction.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 325d9a5f0128..700dc4b34ada 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -402,6 +402,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans, WARN_ON(cur_trans->num_writers < 1); cur_trans->num_writers--; + smp_mb(); if (waitqueue_active(&cur_trans->writer_wait)) wake_up(&cur_trans->writer_wait); put_transaction(cur_trans); @@ -1010,7 +1011,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, struct btrfs_root *root) { unsigned long joined = 0; - unsigned long timeout = 1; struct btrfs_transaction *cur_trans; struct btrfs_transaction *prev_trans = NULL; DEFINE_WAIT(wait); @@ -1081,11 +1081,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, snap_pending = 1; WARN_ON(cur_trans != trans->transaction); - if (cur_trans->num_writers > 1) - timeout = MAX_SCHEDULE_TIMEOUT; - else if (should_grow) - timeout = 1; - mutex_unlock(&root->fs_info->trans_mutex); if (flush_on_commit || snap_pending) { @@ -1107,8 +1102,10 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, TASK_UNINTERRUPTIBLE); smp_mb(); - if (cur_trans->num_writers > 1 || should_grow) - schedule_timeout(timeout); + if (cur_trans->num_writers > 1) + schedule_timeout(MAX_SCHEDULE_TIMEOUT); + else if (should_grow) + schedule_timeout(1); mutex_lock(&root->fs_info->trans_mutex); finish_wait(&cur_trans->writer_wait, &wait); -- 2.20.1