Re: [PATCH] printk: Remove separate printk_sched buffers and use printk buf instead

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 6 May 2014 16:37:14 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:


> What I have in -next is very different from this version of the patch. 
> What's happening?

Hmm, it looks massaged from what I originally sent in order to be
placed after the other patches in the series. A quick review of the
patch seems to be mostly the same logic, but it is a bit different.

Jan, did you update my patch?

> 
> 
> From: Steven Rostedt <rostedt@goodmis.org>
> Subject: printk: remove separate printk_sched buffers and use printk buf instead
> 
> To prevent deadlocks with doing a printk inside the scheduler,
> printk_sched() was created.  The issue is that printk has a console_sem
> that it can grab and release.  The release does a wake up if there's a
> task pending on the sem, and this wake up grabs the rq locks that is held
> in the scheduler.  This leads to a possible deadlock if the wake up uses
> the same rq as the one with the rq lock held already.
> 
> What printk_sched() does is to save the printk write in a per cpu buffer
> and sets the PRINTK_PENDING_SCHED flag.  On a timer tick, if this flag is
> set, the printk() is done against the buffer.
> 
> There's a couple of issues with this approach.
> 
> 1) If two printk_sched()s are called before the tick, the second one
>    will overwrite the first one.
> 
> 2) The temporary buffer is 512 bytes and is per cpu.  This is a quite a
>    bit of space wasted for something that is seldom used.
> 
> In order to remove this, the printk_sched() can use the printk buffer
> instead, and delay the console_trylock()/console_unlock() to the queued
> work.
> 
> Because printk_sched() would then be taking the logbuf_lock, the
> logbuf_lock must not be held while doing anything that may call into the
> scheduler functions, which includes wake ups.  Unfortunately, printk()
> also has a console_sem that it uses, and on release, the up(&console_sem)
> may do a wake up of any pending waiters.  This must be avoided while
> holding the logbuf_lock.
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Jan, if you did massage my patch, you should add here what you did.
Usually in [brackets]. Otherwise it puts the blame on me if something
breaks, or adds confusion if I happen to send out another patch like I
just did.

-- Steve

> Signed-off-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---