linux-kernel - Re: dcache shrink list corruption?

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Message-ID: <20140430190227.GR18016@ZenIV.linux.org.uk>
Date:	Wed, 30 Apr 2014 20:02:27 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: dcache shrink list corruption?

On Wed, Apr 30, 2014 at 08:42:01PM +0200, Miklos Szeredi wrote:

> Message-ID: <20140430154958.GC3113@...sk.piliscsaba.szeredi.hu>

I see...  Several points:
	* I still think that it's better to do handling of "we see
DCACHE_DENTRY_KILLED already set" in dentry_kill() itself.
	* in dentry_kill(dentry, 0) we *know* that it's not on a shrink
list - the caller has just removed it from there and we'd kept ->d_lock
since that point.   What's more, with that scheme we don't need to bother
with can_free at all - just grab ->d_lock after ->d_release() call and check
DCACHE_SHRINK_LIST.  No sense trying to avoid that - in case where we
could just go ahead and free the sucker, there's neither contention nor
anybody else interested in that cacheline, so spin_lock(&dentry->d_lock)
is pretty much free.

IOW, I'd prefer to do the following (completely untested) on top of 1/6--4/6:

diff --git a/fs/dcache.c b/fs/dcache.c
index e482775..82d8eb4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -489,6 +489,17 @@ relock:
 		goto relock;
 	}
 
+	if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
+		if (parent)
+			spin_unlock(&parent->d_lock);
+		if (dentry->d_flags & DCACHE_MAY_FREE) {
+			spin_unlock(&dentry->d_lock);
+			dentry_free(dentry);
+		} else {
+			spin_unlock(&dentry->d_lock);
+		}
+		return parent;
+	}
 	/*
 	 * The dentry is now unrecoverably dead to the world.
 	 */
@@ -504,8 +515,6 @@ relock:
 	if (dentry->d_flags & DCACHE_LRU_LIST) {
 		if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
 			d_lru_del(dentry);
-		else
-			d_shrink_del(dentry);
 	}
 	/* if it was on the hash then remove it */
 	__d_drop(dentry);
@@ -527,7 +536,14 @@ relock:
 	if (dentry->d_op && dentry->d_op->d_release)
 		dentry->d_op->d_release(dentry);
 
-	dentry_free(dentry);
+	spin_lock(&dentry->d_lock);
+	if (dentry->d_flags & DCACHE_SHRINK_LIST) {
+		dentry->d_flags |= DCACHE_MAY_FREE;
+		spin_unlock(&dentry->d_lock);
+	} else {
+		spin_unlock(&dentry->d_lock);
+		dentry_free(dentry);
+	}
 	return parent;
 }
 
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 3b9bfdb..3c7ec32 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -221,6 +221,8 @@ struct dentry_operations {
 #define DCACHE_SYMLINK_TYPE		0x00300000 /* Symlink */
 #define DCACHE_FILE_TYPE		0x00400000 /* Other file type */
 
+#define DCACHE_MAY_FREE			0x00800000
+
 extern seqlock_t rename_lock;
 
 static inline int dname_external(const struct dentry *dentry)
--
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