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

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Message-ID: <20140506070446.33bde17c@gandalf.local.home>
Date:	Tue, 6 May 2014 07:04:46 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>, pmladek@...e.cz,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH] printk: Remove separate printk_sched buffers and use
 printk buf instead

On Tue, 6 May 2014 11:45:57 +0200
Jan Kara <jack@...e.cz> wrote:

> On Mon 05-05-14 19:18:46, Steven Rostedt wrote:
> > 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@...dmis.org>
>   Andrew already has this patch in -mm tree AFAIK...

Ah, and somebody told me that it wasn't in linux-next. But looking at
it now, it seems to be.

Thanks!

-- Steve
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