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Re: [PATCHv3] PowerPC: Fix a race condition when eliding a lock
- From: "Tulio Magno Quites Machado Filho" <tuliom at linux dot vnet dot ibm dot com>
- To: Torvald Riegel <triegel at redhat dot com>
- Cc: carlos at redhat dot com, adhemerval dot zanella at linaro dot org, libc-alpha at sourceware dot org, munroesj at linux dot vnet dot ibm dot com
- Date: Wed, 26 Aug 2015 13:31:10 -0300
- Subject: Re: [PATCHv3] PowerPC: Fix a race condition when eliding a lock
- Authentication-results: sourceware.org; auth=none
- References: <55D742D3 dot 9050600 at redhat dot com> <1440439895-11812-1-git-send-email-tuliom at linux dot vnet dot ibm dot com> <1440507739 dot 27492 dot 183 dot camel at localhost dot localdomain> <87oahv9g27 dot fsf at totoro dot lan> <1440578622 dot 30828 dot 40 dot camel at localhost dot localdomain>
Torvald Riegel <email@example.com> writes:
> On Tue, 2015-08-25 at 19:08 -0300, Tulio Magno Quites Machado Filho
>> Torvald Riegel <firstname.lastname@example.org> writes:
>> > On Mon, 2015-08-24 at 15:11 -0300, Tulio Magno Quites Machado Filho
>> >> + threads, either during lock acquisition, or elision. In order to avoid
>> >> + this evaluation from becoming a data race the access of is_lock_free
>> > It could be a data race because you're not using atomics there, but
>> > that's not the whole point. (We use the term "data race" specifically
>> > to refer to the C11/C++11 memory model concept of the same name.)
>> > You want to ensure the lock elision synchronization scheme, and thus are
>> > moving it inside the txn.
>> Are you complaining about the usage of the term "data race"?
>> If so, what about "race condition"?
> Well, that would be better, but a race condition is not necessarily a
> bad thing. It's usually better to say which execution or interleaving
> you need to avoid, than just saying "race condition".
What about the following?
/* CONCURRENCY NOTES:
The macro expression is_lock_free is read and possibly written to by
multiple threads, either during lock acquisition, or elision. In order to
avoid this evaluation from becoming a race condition with the lock
acquirement from the lock primitive, the access of is_lock_free is placed
*inside* the transaction. Within the transaction we are assured that all
memory accesses are atomic and is_lock_free can be evaluated with relaxed
memory order. That way, the value of is_lock_free is consistent with the
state of the lock until the end of the transaction. */
> I'd still like to see this getting improved eventually, and I'd
> appreciate if you could participate in that.
Ack. Count me in.