From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756796Ab1FUPkq (ORCPT ); Tue, 21 Jun 2011 11:40:46 -0400 Received: from merlin.infradead.org ([205.233.59.134]:52973 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756749Ab1FUPkm (ORCPT ); Tue, 21 Jun 2011 11:40:42 -0400 Message-Id: <20110621153806.132761621@chello.nl> User-Agent: quilt/0.48-1 Date: Tue, 21 Jun 2011 17:17:26 +0200 From: Peter Zijlstra To: Linus Torvalds , Ingo Molnar , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, efault@gmx.de, Peter Zijlstra Subject: [PATCH 1/4] printk: Release console_sem after logbuf_lock References: <20110621151725.705140475@chello.nl> Content-Disposition: inline; filename=printk-up-after-unlock.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. The reason for unlocking the console_sem under the logbuf_lock is that a concurrent printk() might full up the buffer but fail to acquire the console sem, resulting in a missed write to the console until a subsequent console_sem acquire/release cycle. Signed-off-by: Peter Zijlstra --- kernel/printk.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-) Index: linux-2.6/kernel/printk.c =================================================================== --- linux-2.6.orig/kernel/printk.c +++ linux-2.6/kernel/printk.c @@ -782,7 +782,7 @@ static inline int can_use_console(unsign 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(un */ 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(); }