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Re: [Qemu-devel] [RFC 7/8] util/qht: atomically set b->hashes
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From: |
Paolo Bonzini |
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Subject: |
Re: [Qemu-devel] [RFC 7/8] util/qht: atomically set b->hashes |
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Date: |
Mon, 19 Sep 2016 20:37:06 +0200 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 |
On 19/09/2016 20:06, Emilio G. Cota wrote:
> On Mon, Sep 19, 2016 at 16:51:38 +0100, Alex Bennée wrote:
>> > ThreadSanitizer detects a possible race between reading/writing the
>> > hashes. As ordering semantics are already documented for qht we just
>> > need to ensure a race can't tear the hash value so we can use the
>> > relaxed atomic_set/read functions.
> This was discussed here:
>
> https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg03658.html
>
> To reiterate: reading torn hash values is fine, since the retry will
> happen regardless (and all pointers[] remain valid through the RCU
> read-critical section).
True, but C11 says data races are undefined, not merely unspecified.
seqlock-protected data requires a relaxed read and write, because they
are read concurrently in the read and write sides.
Paolo