Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1616868399-82848-1-git-send-email-guoren@kernel.org>
 <1616868399-82848-4-git-send-email-guoren@kernel.org> <YGGGqftfr872/4CU@hirez.programming.kicks-ass.net>
 <CAK8P3a2bNH-1VjsZmZJkvGzzZY=ckaaOK9ZGL-oD0DH4jW-+kQ@mail.gmail.com>
 <YGG3JIBVO0w6W3fg@hirez.programming.kicks-ass.net> <YGG6Ms5Rl0AOJL2i@hirez.programming.kicks-ass.net>
 <CAJF2gTRwd0QpUZumDFUN1J=effv67ucUdsQ96PJwjBhPgJ1npw@mail.gmail.com>
 <CAK8P3a3jpQ7dDiVG0s_DQiL6n_MdnhYHMjqFfJ92JJBJFPQZPQ@mail.gmail.com>
 <CAJF2gTSpnHndT9NkrzvNP6xvqV51_DENwh2BHaduUnGyUE=Jaw@mail.gmail.com>
 <CAK8P3a0DkbM=4oBBhA2DWvzMV7DwN1sqOU8Wa1qFtpd_w7iWmQ@mail.gmail.com>
 <CAJF2gTSGLn7katm6YAtkKWJcQRqw36_yqn+aK1pKUSRM5V1zUg@mail.gmail.com> <CAJF2gTSXY1fCBSZ-Z=8_AcRxoiCOoaNu-5A_JximGJxZY18RzQ@mail.gmail.com>
In-Reply-To: <CAJF2gTSXY1fCBSZ-Z=8_AcRxoiCOoaNu-5A_JximGJxZY18RzQ@mail.gmail.com>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 31 Mar 2021 09:12:05 +0200
Message-ID: <CAK8P3a3EXJorMNFpG0_M3W2KFNrEqyoYBU9Cp-Fr=HJAZ9KGNA@mail.gmail.com>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32
To:     Guo Ren <guoren@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-csky@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Guo Ren <guoren@linux.alibaba.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Anup Patel <anup@brainfault.org>,
        Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Mar 31, 2021 at 8:44 AM Guo Ren <guoren@kernel.org> wrote:
> On Wed, Mar 31, 2021 at 12:18 PM Guo Ren <guoren@kernel.org> wrote:
> > On Tue, Mar 30, 2021 at 3:12 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Tue, Mar 30, 2021 at 4:26 AM Guo Ren <guoren@kernel.org> wrote:

> > > As I understand, this example must not cause a deadlock on
> > > a compliant hardware implementation when the underlying memory
> > > has RsrvEventual behavior, but could deadlock in case of
> > > RsrvNonEventual
> > Thx for the nice explanation:
> >  - RsrvNonEventual - depends on software fall-back mechanisms, and
> > just I'm worried about.
> >  - RsrvEventual - HW would provide the eventual success guarantee.
> In riscv-spec 8.3 Eventual Success of Store-Conditional Instructions
>
> I found:
> "As a consequence of the eventuality guarantee, if some harts in an
> execution environment are
> executing constrained LR/SC loops, and no other harts or devices in
> the execution environment
> execute an unconditional store or AMO to that reservation set, then at
> least one hart will
> eventually exit its constrained LR/SC loop. *** By contrast, if other
> harts or devices continue to
> write to that reservation set, it ***is not guaranteed*** that any
> hart will exit its LR/SC loop.*** "
>
> Seems RsrvEventual couldn't solve the code's problem I've mentioned.

Ok, got it.

        Arnd