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@gmail.com>
Tested-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@vger.kernel.org

2 files changed, 11 insertions, 11 deletions

diff --git a/fs/jffs2/README.Locking b/fs/jffs2/README.Locking
index 3ea36554107f..8918ac905a3b 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 5a2dec2b064c..95d5880a63ee 100644
--- a/fs/jffs2/gc.c
+++ b/fs/jffs2/gc.c
@@ -1296,14 +1296,17 @@ 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",