On Tue 13-05-14 13:29:58, Andrew Morton wrote:
> On Mon, 12 May 2014 17:41:54 -0700 Derek Basehore <dbaseh...@chromium.org>
> wrote:
>
> > If we don't call mutex_acquire at the beginning of console_unblank, we can
> > run
> > into a lockup on the logbuf_lock between console_unlock and printk during
> > panic.
> > What happens in console_unlock is:
> >
> > -locks logbuf_lock
> > -calls mutex_release
> > -which calls printk
> > -which locks logbuf_lock
> >
> > This fixes the problem by calling console_trylock (which calls
> > mutex_acquire)
> > instead of directly accessing the semaphore in console_unblank and moves the
> > mutex_release to after we unlock logbuf_lock in console_unlock (interrupts
> > are
> > still disabled).
>
> Please take a look at linux-next, where this code has changed a lot.
>
> > index 7228258..c599ab5 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2084,7 +2084,6 @@ skip:
> > local_irq_restore(flags);
> > }
> > console_locked = 0;
> > - mutex_release(&console_lock_dep_map, 1, _RET_IP_);
> >
> > /* Release the exclusive_console once it is used */
> > if (unlikely(exclusive_console))
> > @@ -2092,6 +2091,7 @@ skip:
> >
> > raw_spin_unlock(&logbuf_lock);
> >
> > + mutex_release(&console_lock_dep_map, 1, _RET_IP_);
> > up(&console_sem);
> >
> > /*
> > @@ -2137,7 +2137,7 @@ void console_unblank(void)
> > * oops_in_progress is set to 1..
> > */
> > if (oops_in_progress) {
> > - if (down_trylock(&console_sem) != 0)
> > + if (!console_trylock())
> > return;
> > } else
> > console_lock();
>
> Although the code is quite different, an equivalent change appears to
> have been made already. I'm not sure that bugfix was intentional ;)
It was intentional. It would take extra work to not fix the bug when I
was cleaning up those lockdep annotations so I decided to just fix it... ;)
Honza
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
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