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SubjectRe: [PATCH tty-next] n_tty: Fix termios_rwsem lockdep false positive
On 08/12/2013 06:50 AM, Sergey Senozhatsky wrote:
> On (08/12/13 13:28), Artem Savkov wrote:
>> Hi Peter,
>>
>> On Sun, Aug 11, 2013 at 08:04:23AM -0400, Peter Hurley wrote:
>>> Lockdep reports a circular lock dependency between
>>> atomic_read_lock and termios_rwsem [1]. However, a lock
>>> order deadlock is not possible since CPU1 only holds a
>>> read lock which cannot prevent CPU0 from also acquiring
>>> a read lock on the same r/w semaphore.
>>>
>>> Unfortunately, lockdep cannot currently distinguish whether
>>> the locks are read or write for any particular lock graph,
>>> merely that the locks _were_ previously read and/or write.
>>>
>>> Until lockdep is fixed, re-order atomic_read_lock so
>>> termios_rwsem can be dropped and reacquired without
>>> triggering lockdep.
>>
>> Works fine, thanks.
>>
>> Reported-and-tested-by: Artem Savkov <artem.savkov@gmail.com>
>>
>>> Reported-by: Artem Savkov <artem.savkov@gmail.com>
>>> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
>>> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
>>>
>>> [1] Initial lockdep report from Artem Savkov <artem.savkov@gmail.com>
>>>
>>> ======================================================
>>> [ INFO: possible circular locking dependency detected ]
>>> 3.11.0-rc3-next-20130730+ #140 Tainted: G W
>>> -------------------------------------------------------
>>> bash/1198 is trying to acquire lock:
>>> (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
>>>
>>> but task is already holding lock:
>>> (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
>>>
>>> which lock already depends on the new lock.
>>>
>>> the existing dependency chain (in reverse order) is:
>>>
>>> -> #1 (&ldata->atomic_read_lock){+.+...}:
>>> [<ffffffff811111cc>] validate_chain+0x73c/0x850
>>> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
>>> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
>>> [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540
>>> [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
>>> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
>>> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
>>> [<ffffffff811f2702>] SyS_read+0x62/0xa0
>>> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
>>>
>>> -> #0 (&tty->termios_rwsem){++++..}:
>>> [<ffffffff8111064f>] check_prev_add+0x14f/0x590
>>> [<ffffffff811111cc>] validate_chain+0x73c/0x850
>>> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
>>> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
>>> [<ffffffff81d372c1>] down_read+0x51/0xa0
>>> [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
>>> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
>>> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
>>> [<ffffffff811f2702>] SyS_read+0x62/0xa0
>>> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
>>>
>>> other info that might help us debug this:
>>>
>>> Possible unsafe locking scenario:
>>>
>>> CPU0 CPU1
>>> ---- ----
>>> lock(&ldata->atomic_read_lock);
>>> lock(&tty->termios_rwsem);
>>> lock(&ldata->atomic_read_lock);
>>> lock(&tty->termios_rwsem);
>>>
>>> *** DEADLOCK ***
>>>
>>> 2 locks held by bash/1198:
>>> #0: (&tty->ldisc_sem){.+.+.+}, at: [<ffffffff816ade04>] tty_ldisc_ref_wait+0x24/0x60
>>> #1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
>>>
>>> stack backtrace:
>>> CPU: 1 PID: 1198 Comm: bash Tainted: G W 3.11.0-rc3-next-20130730+ #140
>>> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
>>> 0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002
>>> 0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98
>>> ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670
>>> Call Trace:
>>> [<ffffffff81d34074>] dump_stack+0x59/0x7d
>>> [<ffffffff8110ed75>] print_circular_bug+0x105/0x120
>>> [<ffffffff8111064f>] check_prev_add+0x14f/0x590
>>> [<ffffffff81d3ab5f>] ? _raw_spin_unlock_irq+0x4f/0x70
>>> [<ffffffff811111cc>] validate_chain+0x73c/0x850
>>> [<ffffffff8110ae0f>] ? trace_hardirqs_off_caller+0x1f/0x190
>>> [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
>>> [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
>>> [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
>>> [<ffffffff81d372c1>] down_read+0x51/0xa0
>>> [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
>>> [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
>>> [<ffffffff810e4130>] ? try_to_wake_up+0x210/0x210
>>> [<ffffffff816a3bb6>] tty_read+0x86/0xf0
>>> [<ffffffff811f21d3>] vfs_read+0xc3/0x130
>>> [<ffffffff811f2702>] SyS_read+0x62/0xa0
>>> [<ffffffff815e24ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>>> [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
>>> ---
>>> drivers/tty/n_tty.c | 25 +++++++++++--------------
>>> 1 file changed, 11 insertions(+), 14 deletions(-)
>>>
>
> I hate to do this, but isn't it actually my patch posted here
> https://lkml.org/lkml/2013/8/1/510
>
> which was tagged as `wrong'?

Sergey,

My apologies; I was mistaken regarding this problem being a lockdep
regression (although it's still a false positive from lockdep). Once
I had worked around some issues with the nouveau driver, I was able to
reproduce the lockdep report on 3.10.

I included Artem's lockdep report in the changelog because I received
that first, on 30 July.

My patch below is not the same as your patch of 1 Aug. This patch
preserves the protected access of termios.c_cc[VMIN] and termios.c_cc[VTIME]
(via the MIN_CHAR() and TIME_CHAR() macros).

If you'd prefer, I could add to changelog:

Patch based on original posted here https://lkml.org/lkml/2013/8/1/510
by Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

Regards,
Peter Hurley


>>> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
>>> index dd8ae0c..c9a9ddd 100644
>>> --- a/drivers/tty/n_tty.c
>>> +++ b/drivers/tty/n_tty.c
>>> @@ -2122,6 +2122,17 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
>>> if (c < 0)
>>> return c;
>>>
>>> + /*
>>> + * Internal serialization of reads.
>>> + */
>>> + if (file->f_flags & O_NONBLOCK) {
>>> + if (!mutex_trylock(&ldata->atomic_read_lock))
>>> + return -EAGAIN;
>>> + } else {
>>> + if (mutex_lock_interruptible(&ldata->atomic_read_lock))
>>> + return -ERESTARTSYS;
>>> + }
>>> +
>>> down_read(&tty->termios_rwsem);
>>>
>>> minimum = time = 0;
>>> @@ -2141,20 +2152,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
>>> }
>>> }
>>>
>>> - /*
>>> - * Internal serialization of reads.
>>> - */
>>> - if (file->f_flags & O_NONBLOCK) {
>>> - if (!mutex_trylock(&ldata->atomic_read_lock)) {
>>> - up_read(&tty->termios_rwsem);
>>> - return -EAGAIN;
>>> - }
>>> - } else {
>>> - if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
>>> - up_read(&tty->termios_rwsem);
>>> - return -ERESTARTSYS;
>>> - }
>>> - }
>>> packet = tty->packet;
>>>
>>> add_wait_queue(&tty->read_wait, &wait);
>>> --
>>> 1.8.1.2
>>>
>>
>> --
>> Regards,
>> Artem
>>



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