[PATCH] jffs2: Fix lock acquisition order bug in gc path

Josh Cartwright
2012-02-09T18:51:07Z

The locking policy is such that the erase_complete_block spinlock is
nested within the alloc_sem mutex. This fixes a case in which the
acquisition order was erroneously reversed. This issue was caught by
the following lockdep splat:

=======================================================
[ INFO: possible circular locking dependency detected ]
3.0.5 #1
-------------------------------------------------------
jffs2_gcd_mtd6/299 is trying to acquire lock:
(&c->alloc_sem){+.+.+.}, at: [<c01f7714>] jffs2_garbage_collect_pass+0x314/0x890

but task is already holding lock:
(&(&c->erase_completion_lock)->rlock){+.+...}, at: [<c01f7708>] jffs2_garbage_collect_pass+0x308/0x890

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&(&c->erase_completion_lock)->rlock){+.+...}:
[<c008bec4>] validate_chain+0xe6c/0x10bc
[<c008c660>] __lock_acquire+0x54c/0xba4
[<c008d240>] lock_acquire+0xa4/0x114
[<c046780c>] _raw_spin_lock+0x3c/0x4c
[<c01f744c>] jffs2_garbage_collect_pass+0x4c/0x890
[<c01f937c>] jffs2_garbage_collect_thread+0x1b4/0x1cc
[<c0071a68>] kthread+0x98/0xa0
[<c000f264>] kernel_thread_exit+0x0/0x8

-> #0 (&c->alloc_sem){+.+.+.}:
[<c008ad2c>] print_circular_bug+0x70/0x2c4
[<c008c08c>] validate_chain+0x1034/0x10bc
[<c008c660>] __lock_acquire+0x54c/0xba4
[<c008d240>] lock_acquire+0xa4/0x114
[<c0466628>] mutex_lock_nested+0x74/0x33c
[<c01f7714>] jffs2_garbage_collect_pass+0x314/0x890
[<c01f937c>] jffs2_garbage_collect_thread+0x1b4/0x1cc
[<c0071a68>] kthread+0x98/0xa0
[<c000f264>] kernel_thread_exit+0x0/0x8

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&(&c->erase_completion_lock)->rlock);
lock(&c->alloc_sem);
lock(&(&c->erase_completion_lock)->rlock);
lock(&c->alloc_sem);

*** DEADLOCK ***

1 lock held by jffs2_gcd_mtd6/299:
#0: (&(&c->erase_completion_lock)->rlock){+.+...}, at: [<c01f7708>] jffs2_garbage_collect_pass+0x308/0x890

stack backtrace:
[<c00155dc>] (unwind_backtrace+0x0/0x100) from [<c0463dc0>] (dump_stack+0x20/0x24)
[<c0463dc0>] (dump_stack+0x20/0x24) from [<c008ae84>] (print_circular_bug+0x1c8/0x2c4)
[<c008ae84>] (print_circular_bug+0x1c8/0x2c4) from [<c008c08c>] (validate_chain+0x1034/0x10bc)
[<c008c08c>] (validate_chain+0x1034/0x10bc) from [<c008c660>] (__lock_acquire+0x54c/0xba4)
[<c008c660>] (__lock_acquire+0x54c/0xba4) from [<c008d240>] (lock_acquire+0xa4/0x114)
[<c008d240>] (lock_acquire+0xa4/0x114) from [<c0466628>] (mutex_lock_nested+0x74/0x33c)
[<c0466628>] (mutex_lock_nested+0x74/0x33c) from [<c01f7714>] (jffs2_garbage_collect_pass+0x314/0x890)
[<c01f7714>] (jffs2_garbage_collect_pass+0x314/0x890) from [<c01f937c>] (jffs2_garbage_collect_thread+0x1b4/0x1cc)
[<c01f937c>] (jffs2_garbage_collect_thread+0x1b4/0x1cc) from [<c0071a68>] (kthread+0x98/0xa0)
[<c0071a68>] (kthread+0x98/0xa0) from [<c000f264>] (kernel_thread_exit+0x0/0x8)

Signed-off-by: Josh Cartwright <joshc@linux.com>
---
fs/jffs2/gc.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/jffs2/gc.c b/fs/jffs2/gc.c
index 31dce61..4bbd521 100644
--- a/fs/jffs2/gc.c
+++ b/fs/jffs2/gc.c
@@ -225,8 +225,8 @@ int jffs2_garbage_collect_pass(struct jffs2_sb_info *c)
return 0;

D1(printk(KERN_DEBUG "No progress from erasing blocks; doing GC anyway\n"));
- spin_lock(&c->erase_completion_lock);
mutex_lock(&c->alloc_sem);
+ spin_lock(&c->erase_completion_lock);
}

/* First, work out which block we're garbage-collecting */
--
1.7.2.5