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Re: [Qemu-devel] [PATCH 2/5] cpus: use atomic_read to read seqlock-prote
From: |
Emilio G. Cota |
Subject: |
Re: [Qemu-devel] [PATCH 2/5] cpus: use atomic_read to read seqlock-protected variables |
Date: |
Thu, 13 Oct 2016 15:20:15 -0400 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Mon, Oct 10, 2016 at 15:59:04 +0200, Paolo Bonzini wrote:
> There is a data race if the variable is written concurrently to the
> read. In C11 this has undefined behavior. Use atomic_read. The
> write side does not need atomic_set, because it is protected by a
> mutex.
Is tsan happy with the way seqlocks are written right now?
According to this message I just found by Dmitry Vyukov, tsan
shouldn't be. Note however that the message is from April'15,
so it might be outdated:
Dmitry Vyukov wrote:
> 1. Tsan is bad at handling stand-alone memory barriers.
<snip>
> And here is a way to express seqlock that is both correct, is
> understood by tsan and is no overhead on x86:
>
> // writer
> atomic_store(&seq, seq+1, memory_order_relaxed);
> atomic_store(&data[0], ..., memory_order_release);
> ...
> atomic_store(&data[N], ..., memory_order_release);
> atomic_store(&seq, seq+1, memory_order_release);
>
> // reader
> atomic_load(&seq, memory_order_acquire);
> d0 = atomic_load(&data[0], memory_order_acquire);
> ...
> dN = atomic_load(&data[N], memory_order_acquire);
> atomic_load(&seq, memory_order_relaxed);
Source: https://groups.google.com/forum/#!topic/thread-sanitizer/B4i9EMQ4BQE
Thanks,
Emilio
- [Qemu-devel] [PATCH 0/5] More thread sanitizer fixes and atomic.h improvements, Paolo Bonzini, 2016/10/10
- [Qemu-devel] [PATCH 2/5] cpus: use atomic_read to read seqlock-protected variables, Paolo Bonzini, 2016/10/10
- [Qemu-devel] [PATCH 3/5] qemu-thread: use acquire/release to clarify semantics of QemuEvent, Paolo Bonzini, 2016/10/10
- [Qemu-devel] [PATCH 1/5] atomic: introduce smp_mb_acquire and smp_mb_release, Paolo Bonzini, 2016/10/10
- [Qemu-devel] [PATCH 4/5] rcu: simplify memory barriers, Paolo Bonzini, 2016/10/10
- [Qemu-devel] [PATCH 5/5] atomic: base mb_read/mb_set on load-acquire and store-release, Paolo Bonzini, 2016/10/10