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 ;)
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