Messages in this thread | | | Date | Thu, 01 Aug 2013 16:06:15 -0400 | From | Peter Hurley <> | Subject | Re: [PATCH] n_tty: release atomic_read_lock before calling schedule_timeout() |
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On 07/31/2013 07:47 AM, Artem Savkov wrote: > On Tue, Jul 30, 2013 at 12:39:54PM -0400, Peter Hurley wrote: >> On 07/30/2013 11:35 AM, Artem Savkov wrote: >>> ldata->atomic_read_lock should be released before scheduling as well as >>> tty->termios_rwsem, otherwise there is a potential deadlock detected by lockdep >> >> False positive. >> >>> Introduced in "n_tty: Access termios values safely" >>> (9356b535fcb71db494fc434acceb79f56d15bda2 in linux-next.git) >>> >>> [ 16.822058] ====================================================== >>> [ 16.822058] [ INFO: possible circular locking dependency detected ] >>> [ 16.822058] 3.11.0-rc3-next-20130730+ #140 Tainted: G W >>> [ 16.822058] ------------------------------------------------------- >>> [ 16.822058] bash/1198 is trying to acquire lock: >>> [ 16.822058] (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660 >>> [ 16.822058] >>> [ 16.822058] but task is already holding lock: >>> [ 16.822058] (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660 >>> [ 16.822058] >>> [ 16.822058] which lock already depends on the new lock. >>> [ 16.822058] >>> [ 16.822058] >>> [ 16.822058] the existing dependency chain (in reverse order) is: >>> [ 16.822058] >>> -> #1 (&ldata->atomic_read_lock){+.+...}: >>> [ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850 >>> [ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0 >>> [ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0 >>> [ 16.822058] [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540 >>> [ 16.822058] [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660 >>> [ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0 >>> [ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130 >>> [ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0 >>> [ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b >>> [ 16.822058] >>> -> #0 (&tty->termios_rwsem){++++..}: >>> [ 16.822058] [<ffffffff8111064f>] check_prev_add+0x14f/0x590 >>> [ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850 >>> [ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0 >>> [ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0 >>> [ 16.822058] [<ffffffff81d372c1>] down_read+0x51/0xa0 >>> [ 16.822058] [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660 >>> [ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0 >>> [ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130 >>> [ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0 >>> [ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b >>> [ 16.822058] >>> [ 16.822058] other info that might help us debug this: >>> [ 16.822058] >>> [ 16.822058] Possible unsafe locking scenario: >>> [ 16.822058] >>> [ 16.822058] CPU0 CPU1 >>> [ 16.822058] ---- ---- >>> [ 16.822058] lock(&ldata->atomic_read_lock); >>> [ 16.822058] lock(&tty->termios_rwsem); >>> [ 16.822058] lock(&ldata->atomic_read_lock); >>> [ 16.822058] lock(&tty->termios_rwsem); >>> [ 16.822058] >>> [ 16.822058] *** DEADLOCK *** >> >> This situation is not possible since termios_rwsem is a read/write semaphore; >> CPU1 cannot prevent CPU0 from obtaining a read lock on termios_rwsem. > Oops, yes, sorry. > >> This looks like a regression caused by: >> >> commit a51805efae5dda0da66f79268ffcf0715f9dbea4 >> Author: Michel Lespinasse <walken@google.com> >> Date: Mon Jul 8 14:23:49 2013 -0700 >> >> lockdep: Introduce lock_acquire_exclusive()/shared() helper macros > Doesn't seem to be this commit. I see nothing wrong here and just to be > sure I've checked the kernel with this commit reverted. The issue is > still there.
Yes, you're right. Apologies to Michel for the too-hasty blame.
Thanks for the report anyway. I'll track down the lockdep regression as soon as I fix a real deadlock in the nouveau driver that disables lockdep.
Regards, Peter Hurley
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