Re: [PATCH] jfs: possible deadlocks - continue

[Evgeniy Dushistov <dushistov@mail.ru>]

On Sun, Jun 04, 2006 at 04:49:56PM -0500, Dave Kleikamp wrote:
> On Sun, 2006-06-04 at 19:44 +0400, Evgeniy Dushistov wrote:
> > I should add that this happened during boot, when root jfs
> > file system become from ro->rw
> > 
> > I look at code, and see that
> > 1)locks wasn't release in the opposite order in which
> > they were taken
> 
> Why does this matter?
> 
Like "3" it make code more understandable,
reader of code don't have to think why
code looks like:
lock A, lock B unlock A, unlock B
while a normal practice in kernel:
lock A, lock B unlock B unlock A
the goal of change just simplicity and clearness,
and nothing more.


> I think the warning needs to be fixed by introducing mutex_lock_nested
> in some places.  I'll take a look at it.
> 
To avoid incomprehension, previous patch just make code more
understandable and unlock mutex on "error path", it doesn't fix
this warning and I have no idea why it happend:

====================================
[ BUG: possible deadlock detected! ]
------------------------------------
mount/5587 is trying to acquire lock:
 (&jfs_ip->commit_mutex){--..}, at: [<c02f7096>] mutex_lock+0x12/0x15

but task is already holding lock:
 (&jfs_ip->commit_mutex){--..}, at: [<c02f7096>] mutex_lock+0x12/0x15

which could potentially lead to deadlocks!

other info that might help us debug this:
2 locks held by mount/5587:
 #0:  (&inode->i_mutex){--..}, at: [<c02f7096>] mutex_lock+0x12/0x15
 #1:  (&jfs_ip->commit_mutex){--..}, at: [<c02f7096>] mutex_lock+0x12/0x15

stack backtrace:
 [<c0103095>] show_trace+0x16/0x19
 [<c0103562>] dump_stack+0x1a/0x1f
 [<c012ddd7>] __lockdep_acquire+0x6c6/0x907
 [<c012e063>] lockdep_acquire+0x4b/0x63
 [<c02f6f0c>] __mutex_lock_slowpath+0xa4/0x21c
 [<c02f7096>] mutex_lock+0x12/0x15
 [<c01b99be>] jfs_create+0x90/0x2b8
 [<c0161016>] vfs_create+0x91/0xda
 [<c0163939>] open_namei+0x15a/0x5b0
 [<c015326c>] do_filp_open+0x22/0x39
 [<c01541a8>] do_sys_open+0x40/0xbc
 [<c015424d>] sys_open+0x13/0x15
 [<c02f875d>] sysenter_past_esp+0x56/0x8d

-- 
/Evgeniy