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Date: Fri, 28 Jun 2013 10:44:03 +0300
From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Michael Wang <wangyun@...ux.vnet.ibm.com>,
Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...en8.de>,
"Rafael J. Wysocki" <rjw@...k.pl>,
"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
linux-kernel@...r.kernel.org, cpufreq@...r.kernel.org,
linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] cpu hotplug: rework cpu_hotplug locking (was
[LOCKDEP] cpufreq: possible circular locking dependency detected)
On (06/28/13 10:13), Viresh Kumar wrote:
> On 26 June 2013 02:45, Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:
> >
> > [ 60.277396] ======================================================
> > [ 60.277400] [ INFO: possible circular locking dependency detected ]
> > [ 60.277407] 3.10.0-rc7-dbg-01385-g241fd04-dirty #1744 Not tainted
> > [ 60.277411] -------------------------------------------------------
> > [ 60.277417] bash/2225 is trying to acquire lock:
> > [ 60.277422] ((&(&j_cdbs->work)->work)){+.+...}, at: [<ffffffff810621b5>] flush_work+0x5/0x280
> > [ 60.277444]
> > but task is already holding lock:
> > [ 60.277449] (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff81042d8b>] cpu_hotplug_begin+0x2b/0x60
> > [ 60.277465]
> > which lock already depends on the new lock.
>
> Hi Sergey,
>
> Can you try reverting this patch?
>
> commit 2f7021a815f20f3481c10884fe9735ce2a56db35
> Author: Michael Wang <wangyun@...ux.vnet.ibm.com>
> Date: Wed Jun 5 08:49:37 2013 +0000
>
> cpufreq: protect 'policy->cpus' from offlining during __gov_queue_work()
>
Hello,
Yes, this helps, of course, but at the same time it returns the previous
problem -- preventing cpu_hotplug in some places.
I have a bit different (perhaps naive) RFC patch and would like to hear
comments.
The idead is to brake existing lock dependency chain by not holding
cpu_hotplug lock mutex across the calls. In order to detect active
refcount readers or active writer, refcount now may have the following
values:
-1: active writer -- only one writer may be active, readers are blocked
0: no readers/writer
>0: active readers -- many readers may be active, writer is blocked
"blocked" reader or writer goes to wait_queue. as soon as writer finishes
(refcount becomes 0), it wakeups all existing processes in a wait_queue.
reader perform wakeup call only when it sees that pending writer is present
(active_writer is not NULL).
cpu_hotplug lock now only required to protect refcount cmp, inc, dec
operations so it can be changed to spinlock.
The patch has survived the initial beating:
echo 0 > /sys/devices/system/cpu/cpu2/online
echo 0 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu2/online
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 0 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu3/online
echo 0 > /sys/devices/system/cpu/cpu2/online
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 0 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu1/online
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu2/online
echo 1 > /sys/devices/system/cpu/cpu1/online
echo 0 > /sys/devices/system/cpu/cpu3/online
echo 1 > /sys/devices/system/cpu/cpu3/online
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
kernel/cpu.c | 75 ++++++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 55 insertions(+), 20 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 198a388..7fa7b0f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -50,17 +50,25 @@ static int cpu_hotplug_disabled;
#ifdef CONFIG_HOTPLUG_CPU
static struct {
- struct task_struct *active_writer;
- struct mutex lock; /* Synchronizes accesses to refcount, */
- /*
- * Also blocks the new readers during
- * an ongoing cpu hotplug operation.
+ /* Synchronizes accesses to refcount, also blocks the new readers
+ * during an ongoing cpu hotplug operation.
+ */
+ spinlock_t lock;
+ /* -1: active cpu hotplug process
+ * 0: unlocked
+ * >0: active fercount readers
*/
int refcount;
+ struct task_struct *active_writer;
+ /* Wait queue for new refcount readers during an ongoing
+ * cpu hotplug operation.
+ */
+ wait_queue_head_t wait;
} cpu_hotplug = {
- .active_writer = NULL,
- .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
+ .lock = __SPIN_LOCK_INITIALIZER(cpu_hotplug.lock),
.refcount = 0,
+ .active_writer = NULL,
+ .wait = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wait),
};
void get_online_cpus(void)
@@ -68,10 +76,24 @@ void get_online_cpus(void)
might_sleep();
if (cpu_hotplug.active_writer == current)
return;
- mutex_lock(&cpu_hotplug.lock);
- cpu_hotplug.refcount++;
- mutex_unlock(&cpu_hotplug.lock);
+ for (;;) {
+ DECLARE_WAITQUEUE(wait, current);
+
+ spin_lock(&cpu_hotplug.lock);
+ if (++cpu_hotplug.refcount > 0) {
+ spin_unlock(&cpu_hotplug.lock);
+ break;
+ }
+ /* Ongoing cpu hotplug process */
+ cpu_hotplug.refcount--;
+ add_wait_queue(&cpu_hotplug.wait, &wait);
+ __set_current_state(TASK_UNINTERRUPTIBLE);
+
+ spin_unlock(&cpu_hotplug.lock);
+ schedule();
+ remove_wait_queue(&cpu_hotplug.wait, &wait);
+ }
}
EXPORT_SYMBOL_GPL(get_online_cpus);
@@ -79,15 +101,15 @@ void put_online_cpus(void)
{
if (cpu_hotplug.active_writer == current)
return;
- mutex_lock(&cpu_hotplug.lock);
+ spin_lock(&cpu_hotplug.lock);
- if (WARN_ON(!cpu_hotplug.refcount))
+ if (WARN_ON(cpu_hotplug.refcount == 0))
cpu_hotplug.refcount++; /* try to fix things up */
+ cpu_hotplug.refcount--;
- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
-
+ if (unlikely(cpu_hotplug.active_writer))
+ wake_up(&cpu_hotplug.wait);
+ spin_unlock(&cpu_hotplug.lock);
}
EXPORT_SYMBOL_GPL(put_online_cpus);
@@ -118,19 +140,32 @@ static void cpu_hotplug_begin(void)
cpu_hotplug.active_writer = current;
for (;;) {
- mutex_lock(&cpu_hotplug.lock);
- if (likely(!cpu_hotplug.refcount))
+ DECLARE_WAITQUEUE(wait, current);
+
+ spin_lock(&cpu_hotplug.lock);
+ if (likely(--cpu_hotplug.refcount == -1)) {
+ spin_unlock(&cpu_hotplug.lock);
break;
+ }
+ /* Refcount readers present */
+ cpu_hotplug.refcount++;
+ add_wait_queue(&cpu_hotplug.wait, &wait);
__set_current_state(TASK_UNINTERRUPTIBLE);
- mutex_unlock(&cpu_hotplug.lock);
+
+ spin_unlock(&cpu_hotplug.lock);
schedule();
+ remove_wait_queue(&cpu_hotplug.wait, &wait);
}
}
static void cpu_hotplug_done(void)
{
+ spin_lock(&cpu_hotplug.lock);
cpu_hotplug.active_writer = NULL;
- mutex_unlock(&cpu_hotplug.lock);
+ cpu_hotplug.refcount++;
+ spin_unlock(&cpu_hotplug.lock);
+
+ wake_up(&cpu_hotplug.wait);
}
/*
--
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