From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Ellerman Subject: Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks Date: Fri, 03 Jul 2020 20:52:02 +1000 Message-ID: <878sg07twt.fsf@mpe.ellerman.id.au> References: <20200702074839.1057733-1-npiggin@gmail.com> <20200702074839.1057733-6-npiggin@gmail.com> <20200702080219.GB16113@willie-the-truck> <1593685459.r2tfxtfdp6.astroid@bobo.none> <20200702103506.GA16418@willie-the-truck> <1593686722.w9psaqk7yp.astroid@bobo.none> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <1593686722.w9psaqk7yp.astroid@bobo.none> Sender: kvm-ppc-owner@vger.kernel.org To: Nicholas Piggin , Will Deacon Cc: Anton Blanchard , Boqun Feng , kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Waiman Long , Ingo Molnar , Peter Zijlstra , virtualization@lists.linux-foundation.org List-Id: linux-arch.vger.kernel.org Nicholas Piggin writes: > Excerpts from Will Deacon's message of July 2, 2020 8:35 pm: >> On Thu, Jul 02, 2020 at 08:25:43PM +1000, Nicholas Piggin wrote: >>> Excerpts from Will Deacon's message of July 2, 2020 6:02 pm: >>> > On Thu, Jul 02, 2020 at 05:48:36PM +1000, Nicholas Piggin wrote: >>> >> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h >>> >> new file mode 100644 >>> >> index 000000000000..f84da77b6bb7 >>> >> --- /dev/null >>> >> +++ b/arch/powerpc/include/asm/qspinlock.h >>> >> @@ -0,0 +1,20 @@ >>> >> +/* SPDX-License-Identifier: GPL-2.0 */ >>> >> +#ifndef _ASM_POWERPC_QSPINLOCK_H >>> >> +#define _ASM_POWERPC_QSPINLOCK_H >>> >> + >>> >> +#include >>> >> + >>> >> +#define _Q_PENDING_LOOPS (1 << 9) /* not tuned */ >>> >> + >>> >> +#define smp_mb__after_spinlock() smp_mb() >>> >> + >>> >> +static __always_inline int queued_spin_is_locked(struct qspinlock *lock) >>> >> +{ >>> >> + smp_mb(); >>> >> + return atomic_read(&lock->val); >>> >> +} >>> > >>> > Why do you need the smp_mb() here? >>> >>> A long and sad tale that ends here 51d7d5205d338 >>> >>> Should probably at least refer to that commit from here, since this one >>> is not going to git blame back there. I'll add something. >> >> Is this still an issue, though? >> >> See 38b850a73034 (where we added a similar barrier on arm64) and then >> c6f5d02b6a0f (where we removed it). >> > > Oh nice, I didn't know that went away. Thanks for the heads up. Argh! I spent so much time chasing that damn bug in the ipc code. > I'm going to say I'm too scared to remove it while changing the > spinlock algorithm, but I'll open an issue and we should look at > removing it. Sounds good. cheers From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37600 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725915AbgGCKtv (ORCPT ); Fri, 3 Jul 2020 06:49:51 -0400 From: Michael Ellerman Subject: Re: [PATCH 5/8] powerpc/64s: implement queued spinlocks and rwlocks In-Reply-To: <1593686722.w9psaqk7yp.astroid@bobo.none> References: <20200702074839.1057733-1-npiggin@gmail.com> <20200702074839.1057733-6-npiggin@gmail.com> <20200702080219.GB16113@willie-the-truck> <1593685459.r2tfxtfdp6.astroid@bobo.none> <20200702103506.GA16418@willie-the-truck> <1593686722.w9psaqk7yp.astroid@bobo.none> Date: Fri, 03 Jul 2020 20:52:02 +1000 Message-ID: <878sg07twt.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-arch-owner@vger.kernel.org List-ID: To: Nicholas Piggin , Will Deacon Cc: Anton Blanchard , Boqun Feng , kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Waiman Long , Ingo Molnar , Peter Zijlstra , virtualization@lists.linux-foundation.org Message-ID: <20200703105202.FFxpe7en27kmNbtVnYcE3LNuIMrpUQ5XB89-JnWY5RY@z> Nicholas Piggin writes: > Excerpts from Will Deacon's message of July 2, 2020 8:35 pm: >> On Thu, Jul 02, 2020 at 08:25:43PM +1000, Nicholas Piggin wrote: >>> Excerpts from Will Deacon's message of July 2, 2020 6:02 pm: >>> > On Thu, Jul 02, 2020 at 05:48:36PM +1000, Nicholas Piggin wrote: >>> >> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h >>> >> new file mode 100644 >>> >> index 000000000000..f84da77b6bb7 >>> >> --- /dev/null >>> >> +++ b/arch/powerpc/include/asm/qspinlock.h >>> >> @@ -0,0 +1,20 @@ >>> >> +/* SPDX-License-Identifier: GPL-2.0 */ >>> >> +#ifndef _ASM_POWERPC_QSPINLOCK_H >>> >> +#define _ASM_POWERPC_QSPINLOCK_H >>> >> + >>> >> +#include >>> >> + >>> >> +#define _Q_PENDING_LOOPS (1 << 9) /* not tuned */ >>> >> + >>> >> +#define smp_mb__after_spinlock() smp_mb() >>> >> + >>> >> +static __always_inline int queued_spin_is_locked(struct qspinlock *lock) >>> >> +{ >>> >> + smp_mb(); >>> >> + return atomic_read(&lock->val); >>> >> +} >>> > >>> > Why do you need the smp_mb() here? >>> >>> A long and sad tale that ends here 51d7d5205d338 >>> >>> Should probably at least refer to that commit from here, since this one >>> is not going to git blame back there. I'll add something. >> >> Is this still an issue, though? >> >> See 38b850a73034 (where we added a similar barrier on arm64) and then >> c6f5d02b6a0f (where we removed it). >> > > Oh nice, I didn't know that went away. Thanks for the heads up. Argh! I spent so much time chasing that damn bug in the ipc code. > I'm going to say I'm too scared to remove it while changing the > spinlock algorithm, but I'll open an issue and we should look at > removing it. Sounds good. cheers