linux-kernel - Re: [PATCH v3] cpu/hotplug: wait for cpuset_hotplug

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Date:   Thu, 1 Apr 2021 15:06:03 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexey Klimov <aklimov@...hat.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, peterz@...radead.org,
        yury.norov@...il.com, daniel.m.jordan@...cle.com,
        jobaker@...hat.com, audralmitchel@...il.com, arnd@...db.de,
        gregkh@...uxfoundation.org, rafael@...nel.org, tj@...nel.org,
        hannes@...xchg.org, klimov.linux@...il.com
Subject: Re: [PATCH v3] cpu/hotplug: wait for cpuset_hotplug_work to finish
 on cpu onlining

On 03/27/21 22:01, Thomas Gleixner wrote:
> And while you carefully reworded the comment, did you actually read what
> it said and what is says now?
> 
> > -		 * cpu_down() which takes cpu maps lock. cpu maps lock
> > -		 * needs to be held as this might race against in kernel
> > -		 * abusers of the hotplug machinery (thermal management).
> 
> vs.
> 
> > +	 * cpu_down() which takes cpu maps lock. cpu maps lock
> > +	 * needed to be held as this might race against in-kernel
> > +	 * abusers of the hotplug machinery (thermal management).
> 
> The big fat hint is: "cpu maps lock needs to be held as this ...." and
> it still needs to be held for the above loop to work correctly at
> all. See also below.
> 
> So just moving comments blindly around and making them past tense is not
> cutting it. Quite the contrary the comments make no sense anymore. They
> became uncomprehensible word salad.
> 
> Now for the second part of that comment:
> 
> > +      *                                          ....  This is
> > +	 * called under the sysfs hotplug lock, so it is properly
> > +	 * serialized against the regular offline usage.
> 
> So there are two layers of protection:
> 
>    cpu_maps_lock and sysfs hotplug lock
> 
> One protects surprisingly against concurrent sysfs writes and the other
> is required to serialize in kernel usage.
> 
> Now lets look at the original protection:
> 
>    lock(sysfs)
>      lock(cpu_maps)
>        hotplug
>         dev->offline = new_state
>         uevent()
>      unlock(cpu_maps)
>    unlock(sysfs)
> 
> and the one you created:
> 
>    lock(sysfs)
>      lock(cpu_maps)
>        hotplug
>      unlock(cpu_maps)
>      dev->offline = new_state
>      uevent()
>    unlock(sysfs)
> 
> Where is that protection scope change mentioned in the change log and
> where is the analysis that it is correct?

The comment do still need updating though. Its reference to thermal management
is cryptic. I couldn't see who performs hotplug operations in thermal code.

I also think generally that comment is no longer valid after the refactor I did
to 'prevent' in-kernel users from calling cpu_up/down() directly and force them
all to go via device_offline/online() which is wrapped via the add/remove_cpu()

	33c3736ec888 cpu/hotplug: Hide cpu_up/down()

So I think except for suspend/resume/hibernate/kexec, all in-kernel users
should be serialized by lock(hotplug) now. Since uevents() don't matter for
suspend/resume/hiberante/kexec I think moving it outside of lock(cpu_maps) is
fine.

So IIUC in the past we had the race

	userspace			in-kernel users

	lock(hotplug)
	cpuhp_smt_disable()
	   lock(cpu_maps)		cpu_down()
					  lock(cpu_maps)

So they were serialized by cpu_maps lock. But that should not happen now since
in-kernel (except for the aforementioned) users should use remove_cpu().

	userspace			in-kernel users

	lock(hotplug)
	cpuhp_smt_disable()
	   lock(cpu_maps)		remove_cpu()
					  lock(hotplug)
					  device_offline()
					    cpu_down()
					      lock(cpu_maps)

Which forces the serialization at lock(hotplug), which is what
lock_device_hotplug_sysfs() is actually tries to hold.

So I think that race condition should not happen now. Or did I miss something?

The only loophole is that cpu_device_up() could be abused if not caught by
reviewers. I didn't find a way to warn/error if someone other than
cpu_subsys_online() uses it. We rely on a comment explaining it..

I think cpuhp_smt_disable/enable can safely call device_offline/online now.
Although it might still be more efficient to hold lock(cpu_maps) once than
repeatedly in a loop.

If we do that, then cpu_up_down_serialize_trainwrech() can be called from
cpu_device_up/down() which implies !task_frozen.

Can't remember now if Alexey moved the uevent() handling out of the loop for
efficiency reasons or was seeing something else. I doubt it was the latter.

Thanks

--
Qais Yousef