[PATCH 4.2.y-ckt 19/98] jffs2: Fix page lock / f->sem deadlock

Kamal Mostafa
2016-03-15T23:30:13Z

From: David Woodhouse <David.Woodhouse@intel.com>

commit 49e91e7079febe59a20ca885a87dd1c54240d0f1 upstream.

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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
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 5a2dec2..95d5880 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",
--
2.7.0