linux-kernel - Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd

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Message-ID: <20150613180056.GB29379@redhat.com>
Date:	Sat, 13 Jun 2015 20:00:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-mml@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
	the Xen guest code

On 06/13, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > > So we could add tsk->mm_leader or so,
> >
> > No, no, please do not. Just do something like
> >
> > 	for_each_process(p) {
> >
> > 		for_each_thread(p, t) {
>
> So far that's what the for_each_process_thread() iterations I added do, right?

Not really,

> > 			if (t->mm) {
> > 				do_something(t->mm);
> > 				break;
                                ^^^^^

Note this "break". We stop the inner loop right after we find a thread "t"
with ->mm != NULL. In the likely case t == p (group leader) so the inner
loop stops on the 1st iteration.


> > But either way I don't understand what protects this ->mm. Perhaps this needs
> > find_lock_task_mm().
>
> That's indeed a bug: I'll add task_lock()/unlock() before looking at ->mm.

Well, in this particular case we are safe. As Boris explained this is called
from stop_machine(). But sync_global_pgds() is not safe.

> find_lock_task_mm() is not needed IMHO: we have a stable reference to 't', as
> task can only go away via RCU expiry, and all the iterations I added are (supposed
> to) be RCU safe.

Sure, you can do lock/unlock by hand. But find_lock_task_mm() can simplify
the code because it checks subthreads if group_leader->mm == NULL. You can
simply do

	rcu_read_lock();
	for_each_process(p) {
		t = find_lock_task_mm(p);
		if (!t)
			continue;

		do_something(t->mm);
		task_unlock(t);
	}
	rcu_read_unlock();

Oleg.

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