linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 16 Feb 2009 22:45:04 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
 single ipi fallback for smp_call_function_many())

On Mon, 2009-02-16 at 22:32 +0100, Oleg Nesterov wrote:
> > I was about to write a response, but found it to be a justification for
> > the read_barrier_depends() at the end of the loop.
> 
> I forgot to mention I don't understand the read_barrier_depends() at the
> end of the loop as well ;)

Suppose cpu0 adds to csd to cpu1:


 cpu0:                 cpu1:

add entry1
mb();
send ipi
                      run ipi handler
                      read_barrier_depends()
                      while (!list_empty())    [A]
                        do foo

add entry2
mb();
[no ipi -- we still observe entry1]

                        remove foo
                        read_barrier_depends()
                      while (!list_empty())      [B]


The read_barrier_depends() matches the mb() on the other cpu, without
which the 'new' entry might not be observed.

So it turns out the initial one is needed as well.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/