From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
Will Deacon <will@kernel.org>, paulmck <paulmck@kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Andy Lutomirski <luto@amacapital.net>,
Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Alan Stern <stern@rowland.harvard.edu>,
linux-mm <linux-mm@kvack.org>
Subject: Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier
Date: Tue, 4 Aug 2020 13:25:19 -0400 (EDT) [thread overview]
Message-ID: <269299324.40115.1596561919633.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200804165118.GN2657@hirez.programming.kicks-ass.net>
----- On Aug 4, 2020, at 12:51 PM, Peter Zijlstra peterz@infradead.org wrote:
> On Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote:
>> Here is the scenario I have in mind:
>
>> Userspace variables:
>>
>> int x = 0, y = 0;
>>
>> CPU 0 CPU 1
>> Thread A Thread B
>> (in thread group A) (in thread group B)
>>
>> x = 1
>> barrier()
>> y = 1
>> exit()
>> exit_mm()
>> current->mm = NULL;
>> r1 = load y
>> membarrier()
>> skips CPU 0 (no IPI) because its current mm is NULL
>> r2 = load x
>> BUG_ON(r1 == 1 && r2 == 0)
>>
>
> Ah, yes of course.
>
> We really should have a bunch of these scenarios in membarrier.c.
Good point.
>
>
>
> Now, the above cannot happen because we have an unconditional
> atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure
> relying on that is a wee bit dodgy.
I am not against using this already existing barrier to provide the
guarantee we need, but it would have to be documented in the code.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
prev parent reply other threads:[~2020-08-04 17:25 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-28 16:00 [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier Mathieu Desnoyers
2020-08-04 14:34 ` peterz
2020-08-04 14:48 ` Mathieu Desnoyers
2020-08-04 16:51 ` peterz
2020-08-04 17:25 ` Mathieu Desnoyers [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=269299324.40115.1596561919633.JavaMail.zimbra@efficios.com \
--to=mathieu.desnoyers@efficios.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@amacapital.net \
--cc=npiggin@gmail.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=stern@rowland.harvard.edu \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).