Re: [RFC PATCH] shrink_dcache_parent() deadlock

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Tue, Jan 10, 2012 at 2:05 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> I tested Dave's patch and the bug can still be easily reproduced.
>
> And that's to be expected, as the intermediate "being on the lru"
> state that Dave's patch eliminates doesn't play a fundamental part in
> the mechanism of the livelock.  It does eliminate one trylock, but
> that's not the one critical to this bug (removing it is a very good
> idea anyway).
>
> The critical trylock here is the one in dentry_kill() which tries to
> lock the parent.

Ok. Here's your patch munged for current -git. You've got most of a
changelog, can you send this out with the right subject and a
sign-off, and re-test with the current git just to make sure.

Al, do you want to handle this or should I take it directly? I'm
assuming nobody has any objections to Miklos' patch?

                                    Linus

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 fs/dcache.c            |    8 +++++---
 include/linux/dcache.h |    1 +
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 3c6d3113a255..6477af24ff0d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -243,6 +243,7 @@ static void dentry_lru_add(struct dentry *dentry)
 static void __dentry_lru_del(struct dentry *dentry)
 {
 	list_del_init(&dentry->d_lru);
+	dentry->d_flags &= ~DCACHE_SHRINK_LIST;
 	dentry->d_sb->s_nr_dentry_unused--;
 	dentry_stat.nr_unused--;
 }
@@ -806,6 +807,7 @@ relock:
 			spin_unlock(&dentry->d_lock);
 		} else {
 			list_move_tail(&dentry->d_lru, &tmp);
+			dentry->d_flags |= DCACHE_SHRINK_LIST;
 			spin_unlock(&dentry->d_lock);
 			if (!--count)
 				break;
@@ -1098,11 +1100,11 @@ resume:
 		/*
 		 * move only zero ref count dentries to the dispose list.
 		 */
-		if (!dentry->d_count) {
+		if (dentry->d_count) {
+			dentry_lru_del(dentry);
+		} else if(!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
 			dentry_lru_move_list(dentry, dispose);
 			found++;
-		} else {
-			dentry_lru_del(dentry);
 		}
 
 		/*
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index a47bda5f76db..31f73220e7d7 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -203,6 +203,7 @@ struct dentry_operations {
 
 #define DCACHE_CANT_MOUNT	0x0100
 #define DCACHE_GENOCIDE		0x0200
+#define DCACHE_SHRINK_LIST	0x0400
 
 #define DCACHE_NFSFS_RENAMED	0x1000
      /* this dentry has been "silly renamed" and has to be deleted on the last