printk question:why release console_sem after logbuf_lock

[Peter Teoh]

Details u have to look at the source code, my guess is based on the
following posting:

http://kerneltrap.org/mailarchive/linux-kernel/2008/1/23/595569

https://lkml.org/lkml/2011/6/21/249

logic:

a.  u want to be able to do printk from anywhere.
b.  but every call to printk requires a console_sem lock to be acquired.
after acquiring console_sem, printk actually serializes the output to a
memory buffer.
c.  now problem arises when printk is happening very fast, and so this type
of locks is ill-suited for printk().
d.   later than this patch is another attempt:

https://patchwork.kernel.org/patch/1760211/
https://lkml.org/lkml/2012/10/20/90

where lazy irq work is being used instead.

read through the comments in the intro to the patch - it covers a lot more
than i mentioned here.

In Documentation/lockdep_design.txt discuss about using irq tracing to
trace the lock dependencies.

Lock inversion is a common computer science problem....look up wiki.

On Sat, Feb 9, 2013 at 10:55 AM, buyitian <buyit at live.cn> wrote:

>       in the patch 0b5e1c5255e7ee8670e077e8224e5c2281229a5b, it releases
> console_sem after logbuf_lock,  the description of this patch is as below:
>
> Release console_sem after unlocking the logbuf_lock so that we don't
> generate wakeups while holding logbuf_lock. This avoids some lock
> inversion troubles once we remove the lockdep_off bits between
> logbuf_lock and rq->lock (prints while holding rq->lock vs doing
> wakeups while holding logbuf_lock).
> There's of course still an actual deadlock where the printk()s under
> rq->lock will issue a wakeup from the up() call, but lockdep won't
> warn about that since semaphores are not tracked.
>
>       could you please give me a detail example about the issue it tries
> to fix? thanks.
>
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-- 
Regards,
Peter Teoh
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