On Sat, 5 Aug 2006 15:47:30 -0400
Dave Jones <[email protected]> wrote:
> On Sat, Aug 05, 2006 at 12:46:00AM -0700, Andrew Morton wrote:
> > @@ -320,10 +320,10 @@ void fastcall flush_workqueue(struct wor
> > } else {
> > int cpu;
> >
> > - lock_cpu_hotplug();
> > + mutex_lock(&workqueue_mutex);
> > for_each_online_cpu(cpu)
> > flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
> > - unlock_cpu_hotplug();
> > + mutex_unlock(&workqueue_mutex);
> > }
> > }
>
> Is this enough though? I mean, what stops the hotplug cpu code
> from modifying cpu_online_map underneath us here, making for_each_online_cpu
> do the wrong thing ?
>
The patch takes that mutex in workqueue_cpu_callback() in such a way as to
prevent that. What it _effectively_ does is to change cpu_up() thusly:
--- a/kernel/cpu.c~x
+++ a/kernel/cpu.c
@@ -197,6 +197,7 @@ int __devinit cpu_up(unsigned int cpu)
}
ret = blocking_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
+ mutex_lock(&workqueue_mutex);
if (ret == NOTIFY_BAD) {
printk("%s: attempt to bring up CPU %u failed\n",
__FUNCTION__, cpu);
@@ -213,12 +214,15 @@ int __devinit cpu_up(unsigned int cpu)
BUG_ON(!cpu_online(cpu));
/* Now call notifier in preparation. */
+ mutex_unlock(&workqueue_mutex);
blocking_notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu);
out_notify:
- if (ret != 0)
+ if (ret != 0) {
blocking_notifier_call_chain(&cpu_chain,
CPU_UP_CANCELED, hcpu);
+ mutex_unlock(&workqueue_mutex);
+ }
out:
mutex_unlock(&cpu_add_remove_lock);
return ret;
_
and similarly in cpu_down().
So the workqueue code itself is protecting its per-cpu data from the hot
plug/unplug code across its critical regions.
-
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