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Re: [PATCH v2] PowerPC: Fix a race condition when eliding a lock
- From: "Carlos O'Donell" <carlos at redhat dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, Tulio Magno Quites Machado Filho <tuliom at linux dot vnet dot ibm dot com>, triegel at redhat dot com
- Cc: libc-alpha at sourceware dot org, munroesj at linux dot vnet dot ibm dot com
- Date: Fri, 21 Aug 2015 11:22:37 -0400
- Subject: Re: [PATCH v2] PowerPC: Fix a race condition when eliding a lock
- Authentication-results: sourceware.org; auth=none
- References: <55BAEE77 dot 9000501 at redhat dot com> <1440084054-32243-1-git-send-email-tuliom at linux dot vnet dot ibm dot com> <55D628C2 dot 3080305 at redhat dot com> <55D62FD3 dot 8090109 at linaro dot org>
On 08/20/2015 03:51 PM, Adhemerval Zanella wrote:
>
>
>
>>> +
>>> +/* Returns 0 if the lock defined by is_lock_free was elided.
>>> + ADAPT_COUNT is a per-lock state variable. */
>>> +# define ELIDE_LOCK(adapt_count, is_lock_free) \
>>> + ({ \
>>> + int ret = 0; \
>>> + if (adapt_count > 0) \
>>> + (adapt_count)--; \
>>> + else \
>>> + for (int i = __elision_aconf.try_tbegin; i > 0; i--) \
>>> + { \
>>> + if (__builtin_tbegin (0)) \
>>> + { \
>>> + if (is_lock_free) \
>>
>> This should use a relaxed MO atomic access to highlight that this data *is*
>> shared between threads, but that there are no ordering guarantees. It should
>> result in a normal load, but it makes it clear in the code that some other
>> thread is also accessing this data, and that the transaction as a barrier
>> is all that is protecting this from being a data-race (undefined behaviour).
>
> Should we really highlight it using atomic operations? The idea is exactly
> that any access within a transaction will not result in undefined behavior,
> so no need to atomic for synchronization.
I withdraw my suggestion.
I can't come up with a strong enough argument that isn't already solved by
a high quality comment about how the macro works.
Cheers,
Carlos.