Re: [PATCH v2] printk: Avoid softlockups in console_unlock()

Jan Kara
2013-02-06T21:30:05Z

On Wed 06-02-13 12:19:19, Hugh Dickins wrote:
> On Wed, 6 Feb 2013, Jan Kara wrote:
> > On Wed 06-02-13 09:58:48, David Rientjes wrote:
> > > On Wed, 6 Feb 2013, Jan Kara wrote:
> > >
> > > > Yes, I noticed that thread just yesterday and also though that using
> > > > similar trick might be viable. I'll experiment if we could use the same
> > > > method for handling lockup problems I hit. Steven seems to have already
> > > > tweaked PRINTK_PENDING stuff to be usable more easily...
> > > >
> > >
> > > Are these new build failures in linux-next coming from this patch?
> > >
> > > kernel/printk.c: In function 'console_unlock':
> > > kernel/printk.c:2156:18: error: 'printk_work' undeclared (first use in this function)
> > > kernel/printk.c:2156:18: note: each undeclared identifier is reported only once for each function it appears in
> > > kernel/printk.c: At top level:
> > > kernel/printk.c:2167:13: warning: 'printk_worker' defined but not used [-Wunused-function]
> > Yes, I already sent a patch to fix these (attached if you need it).
> > Thanks for notice.
>
> Is the console_unlock patch actually intended to be a console load test?
> Could we just revert it until it's baked?
Yeah, agreed. I'll probably go sligthly different route anyway.

> This little hunk from __console_unlock():
>
> console_may_schedule = 0;
> + cur_cpu = smp_processor_id();
>
> I have CONFIG_PREEMPT=y CONFIG_DEBUG_PREEMPT=y,
> so that fills the console with messages:
Thanks for report, I'll take this in consideration when working on the next
version of the patch. Somehow console_unlock() was never called from the
context you show below in my testing.

> BUG: using smp_processor_id() in preemptible [00000000] code: systemd/1
> caller is __console_unlock+0x3c/0x39c
> Pid: 1, comm: systemd Tainted: G W 3.8.0-rc6-mm1 #1
> Call Trace:
> [<ffffffff81223ee2>] debug_smp_processor_id+0xbe/0xd8
> [<ffffffff810615d8>] __console_unlock+0x3c/0x39c
> [<ffffffff810a6a38>] ? trace_hardirqs_on+0xd/0xf
> [<ffffffff8154a997>] ? _raw_spin_unlock_irqrestore+0x46/0x64
> [<ffffffff81061941>] console_unlock+0x9/0x1b
> [<ffffffff812955ab>] con_shutdown+0x29/0x2d
> [<ffffffff81295582>] ? visual_init+0x10d/0x10d
> [<ffffffff812857ed>] release_tty+0x4d/0x91
> [<ffffffff8128644f>] tty_release+0x421/0x460
> [<ffffffff81120c57>] __fput+0x104/0x1e9
> [<ffffffff81120d45>] ____fput+0x9/0xb
> [<ffffffff8107a6c3>] task_work_run+0x79/0xa6
> [<ffffffff8102f4a6>] do_notify_resume+0x55/0x66
> [<ffffffff8121e29e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> [<ffffffff8154b898>] int_signal+0x12/0x17

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR