linux-kernel - Re: JFFS2 deadlock

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Date:	Mon, 01 Feb 2016 14:28:34 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Thomas.Betker@...de-schwarz.com, computersforpeace@...il.com
Cc:	linux-mtd <linux-mtd-bounces@...ts.infradead.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Thomas Betker <thomas.betker@...enet.de>,
	linux-kernel@...r.kernel.org,
	Joakim Tjernlund <Joakim.Tjernlund@...inera.com>,
	"sztomi89@...il.com" <sztomi89@...il.com>,
	wangzaiwei <wangzaiwei@...-vision.cn>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Ming Liu <liu.ming50@...il.com>, linux-fsdevel@...r.kernel.org,
	Deng Chao <deng.chao1@....com.cn>
Subject: Re: JFFS2 deadlock

On Thu, 2016-01-28 at 09:16 +0100, Thomas.Betker@...de-schwarz.com wrote:
> 
> Subject: [PATCH] Revert "jffs2: Fix lock acquisition order bug in 
> jffs2_write_begin"
> http://article.gmane.org/gmane.linux.drivers.mtd/62951
> 
> This is a patch revising my original patch, which I sent to linux-mtd on 
> 10-Nov-2015. I didn't see a response yet, but it's one of the outstanding 
> patches above.

That looks necessary but not sufficient. I think we need this
(untested) patch on top of it, to ensure that we *always* take the page
lock before f->sem?

Please could you try what's in the tree at
http://git.infradead.org/users/dwmw2/jffs2-fixes.git

----
From: David Woodhouse <David.Woodhouse@...el.com>
Subject: [PATCH] jffs2: Fix page lock / f->sem deadlock

With this fix, all code paths should now be obtaining the page lock before
f->sem.

Reported-by: Szabó Tamás <sztomi89@...il.com>
Reported-by: Thomas Betker <thomas.betker@...de-schwarz.com>
Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
Cc: stable@...r.kernel.org
---
 fs/jffs2/README.Locking |  5 +----
 fs/jffs2/gc.c           | 17 ++++++++++-------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/fs/jffs2/README.Locking b/fs/jffs2/README.Locking
index 3ea3655..8918ac9 100644
--- a/fs/jffs2/README.Locking
+++ b/fs/jffs2/README.Locking
@@ -2,10 +2,6 @@
 	JFFS2 LOCKING DOCUMENTATION
 	---------------------------
 
-At least theoretically, JFFS2 does not require the Big Kernel Lock
-(BKL), which was always helpfully obtained for it by Linux 2.4 VFS
-code. It has its own locking, as described below.
-
 This document attempts to describe the existing locking rules for
 JFFS2. It is not expected to remain perfectly up to date, but ought to
 be fairly close.
@@ -69,6 +65,7 @@ Ordering constraints:
 	   any f->sem held.
 	2. Never attempt to lock two file mutexes in one thread.
 	   No ordering rules have been made for doing so.
+	3. Never lock a page cache page with f->sem held.
 
 
 	erase_completion_lock spinlock
diff --git a/fs/jffs2/gc.c b/fs/jffs2/gc.c
index 6fb0802..5919fef 100644
--- a/fs/jffs2/gc.c
+++ b/fs/jffs2/gc.c
@@ -1316,14 +1316,17 @@ static int jffs2_garbage_collect_dnode(struct jffs2_sb_info *c, struct jffs2_era
 		BUG_ON(start > orig_start);
 	}
 
-	/* First, use readpage() to read the appropriate page into the page cache */
-	/* Q: What happens if we actually try to GC the _same_ page for which commit_write()
-	 *    triggered garbage collection in the first place?
-	 * A: I _think_ it's OK. read_cache_page shouldn't deadlock, we'll write out the
-	 *    page OK. We'll actually write it out again in commit_write, which is a little
-	 *    suboptimal, but at least we're correct.
-	 */
+	/* The rules state that we must obtain the page lock *before* f->sem, so
+	 * drop f->sem temporarily. Since we also hold c->alloc_sem, nothing's
+	 * actually going to *change* so we're safe; we only allow reading.
+	 *
+	 * It is important to note that jffs2_write_begin() will ensure that its
+	 * page is marked Uptodate before allocating space. That means that if we
+	 * end up here trying to GC the *same* page that jffs2_write_begin() is
+	 * trying to write out, read_cache_page() will not deadlock. */
+	mutex_unlock(&f->sem);
 	pg_ptr = jffs2_gc_fetch_page(c, f, start, &pg);
+	mutex_lock(&f->sem);
 
 	if (IS_ERR(pg_ptr)) {
 		pr_warn("read_cache_page() returned error: %ld\n",
-- 
2.5.0

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation


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