From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754339Ab1FJKQd (ORCPT ); Fri, 10 Jun 2011 06:16:33 -0400 Received: from casper.infradead.org ([85.118.1.10]:51983 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752216Ab1FJKQc convert rfc822-to-8bit (ORCPT ); Fri, 10 Jun 2011 06:16:32 -0400 Subject: Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock From: Peter Zijlstra To: Linus Torvalds Cc: Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, efault@gmx.de, Arne Jansen In-Reply-To: <20110609131307.493181962@chello.nl> References: <20110609130647.937204592@chello.nl> <20110609131307.493181962@chello.nl> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 10 Jun 2011 12:15:52 +0200 Message-ID: <1307700952.3941.112.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Subject: printk: Release console_sem after logbuf_lock From: Peter Zijlstra Date: Tue Jun 07 11:15:33 CEST 2011 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. Since console_unlock() needs to flush the buffer while holding console_sem unlocking logbuf_lock before dropping console_sem opens a window in which another cpu could fill the buffer again but wouldn't be able to flush due to us still owning the console_sem, thus loosing a flush in the process. Solve this by dropping logbuf_lock over the up(), but re-acquire it afterwards and check the buffer is still empty, if not try to re-acquire the console_sem and redo the whole flush bit. Signed-off-by: Peter Zijlstra --- kernel/printk.c | 24 +++++++++++++++++++++--- 1 files changed, 21 insertions(+), 3 deletions(-) diff --git a/kernel/printk.c b/kernel/printk.c index 3518539..37dff34 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -782,7 +782,7 @@ static inline int can_use_console(unsigned int cpu) static int console_trylock_for_printk(unsigned int cpu) __releases(&logbuf_lock) { - int retval = 0; + int retval = 0, wake = 0; if (console_trylock()) { retval = 1; @@ -795,12 +795,14 @@ static int console_trylock_for_printk(unsigned int cpu) */ if (!can_use_console(cpu)) { console_locked = 0; - up(&console_sem); + wake = 1; retval = 0; } } printk_cpu = UINT_MAX; spin_unlock(&logbuf_lock); + if (wake) + up(&console_sem); return retval; } static const char recursion_bug_msg [] = @@ -1242,7 +1244,7 @@ void console_unlock(void) { unsigned long flags; unsigned _con_start, _log_end; - unsigned wake_klogd = 0; + unsigned wake_klogd = 0, retry = 0; if (console_suspended) { up(&console_sem); @@ -1251,6 +1253,7 @@ void console_unlock(void) console_may_schedule = 0; +again: for ( ; ; ) { spin_lock_irqsave(&logbuf_lock, flags); wake_klogd |= log_start - log_end; @@ -1271,8 +1274,23 @@ void console_unlock(void) if (unlikely(exclusive_console)) exclusive_console = NULL; + spin_unlock(&logbuf_lock); + up(&console_sem); + + /* + * Someone could have filled up the buffer again, so re-check if there's + * something to flush. In case we cannot trylock the console_sem again, + * there's a new owner and the console_unlock() from them will do the + * flush, no worries. + */ + spin_lock(&logbuf_lock); + if (con_start != log_end) + retry = 1; spin_unlock_irqrestore(&logbuf_lock, flags); + if (retry && console_trylock()) + goto again; + if (wake_klogd) wake_up_klogd(); }