Re: [PATCH] Fix of dcache race leading to busy inodes on umount

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Kirill Korotaev <[email protected]> wrote:

This patch fixes dcache race between shrink_dcache_XXX functions
and dput().

Example race scenario:

	CPU 0				CPU 1
umount /dev/sda1
generic_shutdown_super          shrink_dcache_memory()
shrink_dcache_parent            dput dentry
select_parent                   prune_one_dentry()
                                <<<< child is dead, locks are released,
                                  but parent is still referenced!!! >>>>

skip dentry->parent,
since it's d_count > 0

message: BUSY inodes after umount...
                                <<< parent is left on dentry_unused
				   list, referencing freed super block

We faced these messages about busy inodes constantly after some stress testing with mount/umount operations parrallel with some other activity.
This patch helped the problem.

The patch was heavilly tested on 2.6.8 during 2 months,
this forward-ported version boots and works ok as well.



You've provided no description of how the patch solves the problem.
below

/* dcache_lock protects shrinker's list */
static void shrink_dcache_racecheck(struct dentry *parent, int *racecheck)
{
	struct super_block *sb;
	struct dcache_shrinker *ds;

	sb = parent->d_sb;
	list_for_each_entry(ds, &sb->s_dshrinkers, list) {
		/* is one of dcache shrinkers working on the dentry? */
		if (ds->dentry == parent) {
			*racecheck = 1;
			break;
		}
	}
}




This all looks awfully hacky.  Why is it done this way, and is there no
cleaner solution?

Additional patch info:

This patch solves the race mentioned above via introducing per super block list of dentries which are being held indirectly by it's child which is going away. This usually happens when a child is dput()'ed last time and is holding it's parent reference though it is not in any lists already and can't be found anywhere. To make sure that shrink_dcache_parent() can't shrink dcache tree because of a real reference and not such indirect one we check a race condition by looking through dcache_shrinker list. If the current dentry is found on dcache_shrinker list than it is being referenced indirectly as described above and should be waited for, otherwise we really can't shrink the tree.

Why is it so hacky? We tried to develop a solution which minimally influences perfomance, not slowdowning a fast path and not introducing lock contention via widening existing locks. I'll be happy if someone fixes it more efficiently.

Kirill

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