Date: Sat, 13 Jun 2015 09:26:35 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <Waiman.Long@hp.com>, Thomas Gleixner <tglx@linutronix.de>,
        Denys Vlasenko <dvlasenk@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>, linux-mml@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
 the Xen guest code
Message-ID: <20150613072635.GA30388@gmail.com>
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* Oleg Nesterov <oleg@redhat.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?

> 			if (t->mm) {
> 				do_something(t->mm);
> 				break;
> 			}
> 		}
> 	}
> 
> 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.

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

Thanks,

	Ingo
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