Re: [PATCH 0/2] xfs: kill lockdep false positives from readdir

[Oleg Nesterov <oleg@xxxxxxxxxx>]

forgot to show the patch, sorry for double posting...

On 08/12, Dave Chinner wrote:
>
> Oleg and Jan, this patchset should fix the lockdep issues that have
> been seen with the freeze rework. Oleg, can you you try it with your
> current patchset and testing and let me know if there are any issues
> that you see?

Tested-by: Oleg Nesterov <oleg@xxxxxxxxxx>



To clarify, I tested these patches with

        [PATCH v2 0/8] change sb_writers to use percpu_rw_semaphore

I sent yesterday first. Then I applied the additional patch (attached
below) which just restores the lockdep improvements from v1, everything
looks fine:

        Ran: generic/068 generic/085 generic/280 generic/311 xfs/011 xfs/119 
xfs/297
        Passed all 7 tests

nothing interesting in dmesg.

Thanks Dave!

Oleg.


--- a/fs/super.c
+++ b/fs/super.c
@@ -1215,25 +1215,31 @@ EXPORT_SYMBOL(__sb_start_write);
 static void sb_wait_write(struct super_block *sb, int level)
 {
        percpu_down_write(sb->s_writers.rw_sem + level-1);
-       /*
-        * We are going to return to userspace and forget about this lock, the
-        * ownership goes to the caller of thaw_super() which does unlock.
-        *
-        * FIXME: we should do this before return from freeze_super() after we
-        * called sync_filesystem(sb) and s_op->freeze_fs(sb), and thaw_super()
-        * should re-acquire these locks before s_op->unfreeze_fs(sb). However
-        * this leads to lockdep false-positives, so currently we do the early
-        * release right after acquire.
-        */
-       percpu_rwsem_release(sb->s_writers.rw_sem + level-1, 0, _THIS_IP_);
 }
 
-static void sb_freeze_unlock(struct super_block *sb)
+/*
+ * We are going to return to userspace and forget about these locks, the
+ * ownership goes to the caller of thaw_super()->sb_freeze_acquire().
+ */
+static void sb_freeze_release(struct super_block *sb)
+{
+       int level;
+
+       for (level = SB_FREEZE_LEVELS; --level >= 0; )
+               percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, 
_THIS_IP_);
+}
+
+static void sb_freeze_acquire(struct super_block *sb)
 {
        int level;
 
        for (level = 0; level < SB_FREEZE_LEVELS; ++level)
                percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, 
_THIS_IP_);
+}
+
+static void sb_freeze_unlock(struct super_block *sb)
+{
+       int level;
 
        for (level = SB_FREEZE_LEVELS; --level >= 0; )
                percpu_up_write(sb->s_writers.rw_sem + level);
@@ -1329,6 +1335,7 @@ int freeze_super(struct super_block *sb)
         * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
         */
        sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+       sb_freeze_release(sb);
        up_write(&sb->s_umount);
        return 0;
 }
@@ -1355,11 +1362,14 @@ int thaw_super(struct super_block *sb)
                goto out;
        }
 
+       sb_freeze_acquire(sb);
+
        if (sb->s_op->unfreeze_fs) {
                error = sb->s_op->unfreeze_fs(sb);
                if (error) {
                        printk(KERN_ERR
                                "VFS:Filesystem thaw failed\n");
+                       sb_freeze_release(sb);
                        up_write(&sb->s_umount);
                        return error;
                }