From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934866AbaEFLEw (ORCPT ); Tue, 6 May 2014 07:04:52 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.228]:30368 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934579AbaEFLEu (ORCPT ); Tue, 6 May 2014 07:04:50 -0400 Date: Tue, 6 May 2014 07:04:46 -0400 From: Steven Rostedt To: Jan Kara Cc: LKML , Andrew Morton , Frederic Weisbecker , pmladek@suse.cz, Peter Zijlstra , John Stultz , Paul Gortmaker Subject: Re: [PATCH] printk: Remove separate printk_sched buffers and use printk buf instead Message-ID: <20140506070446.33bde17c@gandalf.local.home> In-Reply-To: <20140506094557.GA9291@quack.suse.cz> References: <20140505191846.35aa6f9b@gandalf.local.home> <20140506094557.GA9291@quack.suse.cz> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.22; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.130:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 6 May 2014 11:45:57 +0200 Jan Kara 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 > 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