From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752318AbbKKKf1 (ORCPT ); Wed, 11 Nov 2015 05:35:27 -0500 Received: from mail-ob0-f193.google.com ([209.85.214.193]:35474 "EHLO mail-ob0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752100AbbKKKfZ (ORCPT ); Wed, 11 Nov 2015 05:35:25 -0500 Date: Wed, 11 Nov 2015 18:34:56 +0800 From: Boqun Feng To: Oleg Nesterov Cc: Peter Zijlstra , mingo@kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, corbet@lwn.net, mhocko@kernel.org, dhowells@redhat.com, torvalds@linux-foundation.org, will.deacon@arm.com Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Message-ID: <20151111103456.GB6314@fixme-laptop.cn.ibm.com> References: <20151102132901.157178466@infradead.org> <20151102134941.005198372@infradead.org> <20151103175958.GA4800@redhat.com> <20151111093939.GA6314@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="uQr8t48UFsdbeI+V" Content-Disposition: inline In-Reply-To: <20151111093939.GA6314@fixme-laptop.cn.ibm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --uQr8t48UFsdbeI+V Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Nov 11, 2015 at 05:39:40PM +0800, Boqun Feng wrote: > Hi Oleg, >=20 > On Tue, Nov 03, 2015 at 06:59:58PM +0100, Oleg Nesterov wrote: > [snip] > >=20 > > Unfortunately this doesn't look exactly right... > >=20 > > spin_unlock_wait() is not equal to "while (locked) relax", the latter > > is live-lockable or at least sub-optimal: we do not really need to spin >=20 > Just be curious, should spin_unlock_wait() semantically be an ACQUIRE? Hmm.. I guess I was wrong, it doesn't need to be an ACQUIRE, it needs only to use the control dependency to order the load of lock state and stores following it. > Because spin_unlock_wait() is used for us to wait for a certain lock to > RELEASE so that we can do something *after* we observe the RELEASE. > Considering the follow example: >=20 > CPU 0 CPU 1 > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D > { X =3D 0 } > WRITE_ONCE(X, 1); > spin_unlock(&lock); > spin_unlock_wait(&lock) > r1 =3D READ_ONCE(X); >=20 > If spin_unlock_wait() is not an ACQUIRE, r1 can be 0 in this case, > right? Am I missing something subtle here? Or spin_unlock_wait() itself > doesn't have the ACQUIRE semantics, but it should always come with a > smp_mb() *following* it to achieve the ACQUIRE semantics? However in > do_exit(), an smp_mb() is preceding raw_spin_unlock_wait() rather than > following, which makes me confused... could you explain that? Thank you > ;-) >=20 But still, there is one suspicious use of smp_mb() in do_exit(): /* * The setting of TASK_RUNNING by try_to_wake_up() may be delayed * when the following two conditions become true. * - There is race condition of mmap_sem (It is acquired by * exit_mm()), and * - SMI occurs before setting TASK_RUNINNG. * (or hypervisor of virtual machine switches to other guest) * As a result, we may become TASK_RUNNING after becoming TASK_DEAD * * To avoid it, we have to wait for releasing tsk->pi_lock which * is held by try_to_wake_up() */ smp_mb(); raw_spin_unlock_wait(&tsk->pi_lock); /* causes final put_task_struct in finish_task_switch(). */ tsk->state =3D TASK_DEAD; tsk->flags |=3D PF_NOFREEZE; /* tell freezer to ignore us */ schedule(); Seems like smp_mb() doesn't need here? Because the control dependency already orders load of tsk->pi_lock and store of tsk->state, and this control dependency order guarantee pairs with the spin_unlock(->pi_lock) in try_to_wake_up() to avoid data race on ->state. Regards, Boqun > Regards, > Boqun >=20 > > until we observe !spin_is_locked(), we only need to synchronize with the > > current owner of this lock. Once it drops the lock we can proceed, we > > do not care if another thread takes the same lock right after that. > >=20 > > Oleg. > >=20 --uQr8t48UFsdbeI+V Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAABCAAGBQJWQxnNAAoJEEl56MO1B/q4RQcIALLJ0nMEYnIYlxQg5kvcT3BW IlN0XZelZLizuezk1Y8MHA+tqTnqLZzIXLpwa//bhoYeKVQrZ2uCe17y7MNidLw2 ryNotftzEmCkX/1rHuFp6EctmAwaRneaDDdDR1JcJ8G1hwXIWJk6Ra8SJrBD0tpu 7N9iVzMDDKSmJ361S6UA+VefnxzTt9fAHAJp0D9LicfnrMqOcE0Om7+bJWFl4crb r4sZDWqXv8UXFdusRdh/btZ7hd94mZA4LPG4HPE67/d4OZaz5buWLkBENHDTtDJQ pmYgu54dAngUEANq1FPMARX5GBCijgeRRKFg+NkvJx0hK8GqqFPAFzxcn55a48A= =ShP3 -----END PGP SIGNATURE----- --uQr8t48UFsdbeI+V--