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From: Arnd Bergmann <arnd@arndb.de>
To: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	umgwanakikbuti@gmail.com, fweisbec@gmail.com,
	akpm@linux-foundation.org, srao@redhat.com, lwoodman@redhat.com,
	atheurer@redhat.com, oleg@redhat.com,
	Ingo Molnar <mingo@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH] sched, time: cmpxchg does not work on 64-bit variable
Date: Tue, 30 Sep 2014 13:56:37 +0200	[thread overview]
Message-ID: <2547036.UshV4pXvhf@wuerfel> (raw)

A recent change to update the stime/utime members of task_struct
using atomic cmpxchg broke configurations on 32-bit machines with
CONFIG_VIRT_CPU_ACCOUNTING_GEN set, because that uses 64-bit
nanoseconds, leading to a link-time error:

kernel/built-in.o: In function `cputime_adjust':
:(.text+0x25234): undefined reference to `__bad_cmpxchg'

This reverts the change that caused the problem, I suspect the real fix
is to conditionally use cmpxchg64 instead, but I have not checked if
that will work on all architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: eb1b4af0a64a ("sched, time: Atomically increment stime & utime")
---
found in ARM randconfig builds on linux-next

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 64492dff8a81..e99e7e54131c 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -603,12 +603,9 @@ static void cputime_adjust(struct task_cputime *curr,
 	 * If the tick based count grows faster than the scheduler one,
 	 * the result of the scaling may go backward.
 	 * Let's enforce monotonicity.
-	 * Atomic exchange protects against concurrent cputime_adjust().
 	 */
-	while (stime > (rtime = ACCESS_ONCE(prev->stime)))
-		cmpxchg(&prev->stime, rtime, stime);
-	while (utime > (rtime = ACCESS_ONCE(prev->utime)))
-		cmpxchg(&prev->utime, rtime, utime);
+	prev->stime = max(prev->stime, stime);
+	prev->utime = max(prev->utime, utime);
 
 out:
 	*ut = prev->utime;


WARNING: multiple messages have this Message-ID (diff)
From: arnd@arndb.de (Arnd Bergmann)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] sched, time: cmpxchg does not work on 64-bit variable
Date: Tue, 30 Sep 2014 13:56:37 +0200	[thread overview]
Message-ID: <2547036.UshV4pXvhf@wuerfel> (raw)

A recent change to update the stime/utime members of task_struct
using atomic cmpxchg broke configurations on 32-bit machines with
CONFIG_VIRT_CPU_ACCOUNTING_GEN set, because that uses 64-bit
nanoseconds, leading to a link-time error:

kernel/built-in.o: In function `cputime_adjust':
:(.text+0x25234): undefined reference to `__bad_cmpxchg'

This reverts the change that caused the problem, I suspect the real fix
is to conditionally use cmpxchg64 instead, but I have not checked if
that will work on all architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: eb1b4af0a64a ("sched, time: Atomically increment stime & utime")
---
found in ARM randconfig builds on linux-next

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 64492dff8a81..e99e7e54131c 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -603,12 +603,9 @@ static void cputime_adjust(struct task_cputime *curr,
 	 * If the tick based count grows faster than the scheduler one,
 	 * the result of the scaling may go backward.
 	 * Let's enforce monotonicity.
-	 * Atomic exchange protects against concurrent cputime_adjust().
 	 */
-	while (stime > (rtime = ACCESS_ONCE(prev->stime)))
-		cmpxchg(&prev->stime, rtime, stime);
-	while (utime > (rtime = ACCESS_ONCE(prev->utime)))
-		cmpxchg(&prev->utime, rtime, utime);
+	prev->stime = max(prev->stime, stime);
+	prev->utime = max(prev->utime, utime);
 
 out:
 	*ut = prev->utime;

             reply	other threads:[~2014-09-30 11:57 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-30 11:56 Arnd Bergmann [this message]
2014-09-30 11:56 ` [PATCH] sched, time: cmpxchg does not work on 64-bit variable Arnd Bergmann
2014-09-30 12:34 ` Rik van Riel
2014-09-30 12:34   ` Rik van Riel
2014-09-30 12:43   ` Dietmar Eggemann
2014-09-30 12:43     ` Dietmar Eggemann
2014-09-30 13:26   ` Arnd Bergmann
2014-09-30 13:26     ` Arnd Bergmann
2014-09-30 13:37   ` Peter Zijlstra
2014-09-30 13:37     ` Peter Zijlstra
2014-09-30 19:59     ` [PATCH v2] sched, time: fix build error with 64 bit cputime_t on 32 bit systems Rik van Riel
2014-09-30 19:59       ` Rik van Riel
2014-09-30 20:07       ` Arnd Bergmann
2014-09-30 20:07         ` Arnd Bergmann
2014-10-03  5:27       ` [tip:sched/core] sched, time: Fix " tip-bot for Rik van Riel
2014-09-30 17:40 ` [PATCH] sched, time: fix " Rik van Riel
2014-09-30 17:40   ` Rik van Riel
2014-09-30 18:49   ` Arnd Bergmann
2014-09-30 18:49     ` Arnd Bergmann
2014-09-30 19:19     ` Rik van Riel
2014-09-30 19:19       ` Rik van Riel
2014-09-30 19:06   ` Frederic Weisbecker
2014-09-30 19:06     ` Frederic Weisbecker
2014-09-30 19:08     ` Peter Zijlstra
2014-09-30 19:08       ` Peter Zijlstra

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