Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix anon_vma memory ordering

From: Nadav Amit <nadav.amit@gmail.com>
To: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Will Deacon <will@kernel.org>, Jann Horn <jannh@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Andrea Parri <parri.andrea@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Akira Yokosawa <akiyks@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix anon_vma memory ordering
Date: Thu, 27 Jul 2023 13:11:43 -0700	[thread overview]
Message-ID: <2229115A-23F2-4B6B-800C-7182199DF79D@gmail.com> (raw)
In-Reply-To: <CAHk-=wgeMOvL=ko_eruf5St7h0e9dVn4+GMKjcdshTJoxyD8tw@mail.gmail.com>

> On Jul 27, 2023, at 12:39 PM, Linus Torvalds <torvalds@linuxfoundation.org> wrote:
> 
> On Thu, 27 Jul 2023 at 12:10, Nadav Amit <nadav.amit@gmail.com> wrote:
>> 
>> Interesting. I wonder if you considered adding to READ_ONCE() something
>> like:
>> 
>>        asm volatile("" : "+g" (x) );
>> 
>> So later loads (such as baz = *ptr) would reload the updated value.
> 
> Not necessarily a bad idea.  Although I suspect you'd want to add
> *two* of them - on either side - to make sure any previous loads
> wouldn't be moved around it either.

You are right, two are needed.

I’ll give it a shot and see if I see changes to the binary.