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Date: Fri, 31 Jul 2015 16:17:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] stop_machine: kill stop_cpus_lock and
lg_double_lock/unlock()
On Fri, Jul 31, 2015 at 01:12:46PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 30, 2015 at 11:55:27PM +0200, Peter Zijlstra wrote:
> >
> > Exclusion between stop_{one,two}_cpu{,s}() and stop_cpus() makes this
> > trivially go away.
> >
> > Paul's RCU branch already kills try_stop_cpus() dead, so that wart is
> > also gone. But we're still stuck with stop_machine_from_inactive_cpu()
> > which does a spin-wait for exclusive state. So I suppose we'll have to
> > keep stop_cpus_mutex :/
>
> *groan* we really need to kill that from_inactive_cpu() shite too. Lemme
> go have a look at how this MTRR crap works.
*sigh* we can't get rid of it :/ The hardware is 'broken', see:
d0af9eed5aa9 ("x86, pat/mtrr: Rendezvous all the cpus for MTRR/PAT
init")
And that means we must not ever block and the global primitive must be a
spinner.
The below is the best we can do.. At least it localizes the ugly.
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -20,7 +20,6 @@
#include <linux/kallsyms.h>
#include <linux/smpboot.h>
#include <linux/atomic.h>
-#include <linux/lglock.h>
/*
* Structure to determine completion condition and record errors. May
@@ -47,14 +46,6 @@ struct cpu_stopper {
static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
static bool stop_machine_initialized = false;
-/*
- * Avoids a race between stop_two_cpus and global stop_cpus, where
- * the stoppers could get queued up in reverse order, leading to
- * system deadlock. Using an lglock means stop_two_cpus remains
- * relatively cheap.
- */
-DEFINE_STATIC_LGLOCK(stop_cpus_lock);
-
static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
{
memset(done, 0, sizeof(*done));
@@ -74,20 +65,25 @@ static void cpu_stop_signal_done(struct
}
/* queue @work to @stopper. if offline, @work is completed immediately */
-static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
+static void __cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
{
struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
- unsigned long flags;
-
- spin_lock_irqsave(&stopper->lock, flags);
-
if (stopper->enabled) {
list_add_tail(&work->list, &stopper->works);
wake_up_process(stopper->thread);
- } else
+ } else {
cpu_stop_signal_done(work->done, false);
+ }
+}
+static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
+{
+ struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
+ unsigned long flags;
+
+ spin_lock_irqsave(&stopper->lock, flags);
+ __cpu_stop_queue_work(cpu, work);
spin_unlock_irqrestore(&stopper->lock, flags);
}
@@ -226,9 +222,14 @@ static int multi_cpu_stop(void *data)
*/
int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
{
- struct cpu_stop_done done;
+ struct cpu_stopper *stopper1, *stopper2;
struct cpu_stop_work work1, work2;
struct multi_stop_data msdata;
+ struct cpu_stop_done done;
+ unsigned long flags;
+
+ if (cpu2 < cpu1)
+ swap(cpu1, cpu2);
preempt_disable();
msdata = (struct multi_stop_data){
@@ -260,10 +261,17 @@ int stop_two_cpus(unsigned int cpu1, uns
return -ENOENT;
}
- lg_double_lock(&stop_cpus_lock, cpu1, cpu2);
- cpu_stop_queue_work(cpu1, &work1);
- cpu_stop_queue_work(cpu2, &work2);
- lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);
+ stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
+ stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
+
+ spin_lock_irqsave(&stopper1->lock, flags);
+ spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
+
+ __cpu_stop_queue_work(cpu1, &work1);
+ __cpu_stop_queue_work(cpu2, &work2);
+
+ spin_unlock(&stopper2->lock);
+ spin_unlock_irqrestore(&stopper1->lock, flags);
preempt_enable();
@@ -304,19 +312,24 @@ static void queue_stop_cpus_work(const s
unsigned int cpu;
/*
- * Disable preemption while queueing to avoid getting
- * preempted by a stopper which might wait for other stoppers
- * to enter @fn which can lead to deadlock.
+ * Disgusting, but take all relevant per-cpu spinlocks to serialize
+ * against stop_{one,two}_cpu{,s}().
*/
- lg_global_lock(&stop_cpus_lock);
+ preempt_disable();
+ for_each_cpu(cpu, cpumask)
+ arch_spin_lock((arch_spinlock_t *)&per_cpu(cpu_stopper.lock, cpu));
+
for_each_cpu(cpu, cpumask) {
work = &per_cpu(cpu_stopper.stop_work, cpu);
work->fn = fn;
work->arg = arg;
work->done = done;
- cpu_stop_queue_work(cpu, work);
+ __cpu_stop_queue_work(cpu, work);
}
- lg_global_unlock(&stop_cpus_lock);
+
+ for_each_cpu(cpu, cpumask)
+ arch_spin_unlock((arch_spinlock_t *)&per_cpu(cpu_stopper.lock, cpu));
+ preempt_enable();
}
static int __stop_cpus(const struct cpumask *cpumask,
--
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