On Fri, Jun 19, 2020 at 08:58:10AM +1000, Dave Chinner wrote:
On Thu, Jun 18, 2020 at 01:19:41PM -0400, Waiman Long wrote:I should have added this:
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.cThe lockdep splat is detailed in the commit message - it most
index 379cbff438bc..1b94b9bfa4d7 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -913,11 +913,33 @@ xfs_fs_freeze(
struct super_block *sb)
{
struct xfs_mount *mp = XFS_M(sb);
+ unsigned long pflags;
+ int ret;
+ /*
+ * A fs_reclaim pseudo lock is added to check for potential deadlock
+ * condition with fs reclaim. The following lockdep splat was hit
+ * occasionally. This is actually a false positive as the allocation
+ * is being done only after the frozen filesystem is no longer dirty.
+ * One way to avoid this splat is to add GFP_NOFS to the affected
+ * allocation calls. This is what PF_MEMALLOC_NOFS is for.
+ *
+ * CPU0 CPU1
+ * ---- ----
+ * lock(sb_internal);
+ * lock(fs_reclaim);
+ * lock(sb_internal);
+ * lock(fs_reclaim);
+ *
+ * *** DEADLOCK ***
+ */
definitely does not need to be repeated in full here because:
a) it doesn't explain why the splat occurring is, and
b) we most definitely don't care about how the lockdep check
that triggered it is implemented.
c) a lot of people don't understand what lockdep reports
are telling them is a problem.
I get a lot of questions like "I saw this lockdep thing, but I can't
work out what it actually means, so can you have a look at it
Dave?". Hence I think directly quoting something people tend not to
understand to explain the problem they didn't understand isn't the
best approach to improving understanding of the problem...