From: "Emilio G. Cota" <cota@braap.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org,
"Peter Crosthwaite" <crosthwaite.peter@gmail.com>,
"Richard Henderson" <rth@twiddle.net>,
"Eduardo Habkost" <ehabkost@redhat.com>,
"Alex Bennée" <alex.bennee@linaro.org>
Subject: Re: [Qemu-devel] [PATCH 1/6] qsp: drop atomics when using the seqlock
Date: Mon, 10 Sep 2018 11:44:42 -0400 [thread overview]
Message-ID: <20180910154442.GD19941@flamenco> (raw)
In-Reply-To: <e0055fc2-778d-c9cd-c157-29032085e2e7@redhat.com>
On Mon, Sep 10, 2018 at 01:32:15 +0200, Paolo Bonzini wrote:
> On 03/09/2018 19:18, Emilio G. Cota wrote:
> > Using atomics here is a mistake since they're not guaranteed
> > to compile.
>
> But isn't it technically a C11 data race if you don't use atomics?
Yes, it's undefined behaviour.
> Could we make nocheck read/set degrade to just a volatile access when
> used on a variable that is bigger than pointers, or perhaps always
> except when using tsan?
But volatile wouldn't save you from undefined behaviour, would it?
A simpler and definitely correct alternative is to just use a
spinlock instead of the seqlock also for reads when !CONFIG_ATOMIC64.
We don't care about scalability on those rare hosts anyway, so
I'd go with that.
Thanks,
Emilio
next prev parent reply other threads:[~2018-09-10 15:44 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-03 17:18 [Qemu-devel] [PATCH 0/6] i386 + x86_64 mttcg Emilio G. Cota
2018-09-03 17:18 ` [Qemu-devel] [PATCH 1/6] qsp: drop atomics when using the seqlock Emilio G. Cota
2018-09-09 23:32 ` Paolo Bonzini
2018-09-10 15:44 ` Emilio G. Cota [this message]
2018-09-11 11:11 ` Paolo Bonzini
2018-09-03 17:18 ` [Qemu-devel] [PATCH 2/6] test-rcu-list: avoid torn accesses to n_reclaims and n_nodes_removed Emilio G. Cota
2018-09-04 17:37 ` Murilo Opsfelder Araujo
2018-09-04 19:35 ` Emilio G. Cota
2018-09-04 20:56 ` Murilo Opsfelder Araujo
2018-09-04 21:32 ` Emilio G. Cota
2018-09-11 11:25 ` Paolo Bonzini
2018-09-03 17:18 ` [Qemu-devel] [PATCH 3/6] atomic: fix comment s/x64_64/x86_64/ Emilio G. Cota
2018-09-10 9:12 ` Alex Bennée
2018-09-03 17:18 ` [Qemu-devel] [PATCH 4/6] cpus: initialize timers_state.vm_clock_lock Emilio G. Cota
2018-09-10 9:13 ` Alex Bennée
2018-09-03 17:18 ` [Qemu-devel] [PATCH 5/6] target/i386/translate: use thread-local storage in !user-mode Emilio G. Cota
2018-09-10 9:17 ` Alex Bennée
2018-09-10 12:30 ` Emilio G. Cota
2018-09-10 13:43 ` Alex Bennée
2018-09-11 11:24 ` Paolo Bonzini
2018-09-11 17:21 ` Emilio G. Cota
2018-09-03 17:18 ` [Qemu-devel] [PATCH 6/6] configure: enable mttcg for i386 and x86_64 Emilio G. Cota
2018-09-11 11:25 ` [Qemu-devel] [PATCH 0/6] i386 + x86_64 mttcg Paolo Bonzini
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