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authorBrian Foster <bfoster@redhat.com>2015-07-29 11:51:01 +1000
committerDave Chinner <david@fromorbit.com>2015-07-29 11:51:01 +1000
commit4703da7b78776140477a023c99683d3be84b7fca (patch)
treef24d13dde915a288b5c31a1aa7518d90d6429b5e
parentab7bb61092308e83130b8d15725aee1672991d65 (diff)
xfs: close xc_cil list_empty() races with cil commit sequence
We have seen somewhat rare reports of the following assert from xlog_cil_push_background() failing during ltp tests or somewhat innocuous desktop root fs workloads (e.g., virt operations, initramfs construction): ASSERT(!list_empty(&cil->xc_cil)); The reasoning behind the assert is that the transaction has inserted items to the CIL and hit background push codepath all with cil->xc_ctx_lock held for reading. This locks out background commit from emptying the CIL, which acquires the lock for writing. Therefore, the reasoning is that the items previously inserted in the CIL should still be present. The cil->xc_ctx_lock read lock is not sufficient to protect the xc_cil list, however, due to how CIL insertion is handled. xlog_cil_insert_items() inserts and reorders the dirty transaction items to the tail of the CIL under xc_cil_lock. It uses list_move_tail() to achieve insertion and reordering in the same block of code. This function removes and reinserts an item to the tail of the list. If a transaction commits an item that was already logged and thus already resides in the CIL, and said item is the sole item on the list, the removal and reinsertion creates a temporary state where the list is actually empty. This state is not valid and thus should never be observed by concurrent transaction commit-side checks in the circumstances outlined above. We do not want to acquire the xc_cil_lock in all of these instances as it was previously removed and replaced with a separate push lock for performance reasons. Therefore, close any races with list_empty() on the insertion side by ensuring that the list is never in a transient empty state. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>
-rw-r--r--fs/xfs/xfs_log_cil.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index abc2ccbff739..4e7649351f5a 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -307,7 +307,13 @@ xlog_cil_insert_items(
if (!(lidp->lid_flags & XFS_LID_DIRTY))
continue;
- list_move_tail(&lip->li_cil, &cil->xc_cil);
+ /*
+ * Only move the item if it isn't already at the tail. This is
+ * to prevent a transient list_empty() state when reinserting
+ * an item that is already the only item in the CIL.
+ */
+ if (!list_is_last(&lip->li_cil, &cil->xc_cil))
+ list_move_tail(&lip->li_cil, &cil->xc_cil);
}
/* account for space used by new iovec headers */