Messages in this thread | | | Date | Wed, 13 Feb 2019 08:45:44 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock() |
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* Waiman Long <longman@redhat.com> wrote:
> I looked at the assembly code in arch/x86/include/asm/rwsem.h. For both > trylocks (read & write), the count is read first before attempting to > lock it. We did the same for all trylock functions in other locks. > Depending on how the trylock is used and how contended the lock is, it > may help or hurt performance. Changing down_read_trylock to do an > unconditional cmpxchg will change the performance profile of existing > code. So I would prefer keeping the current code. > > I do notice now that the generic down_write_trylock() code is doing an > unconditional compxchg. So I wonder if we should change it to read the > lock first like other trylocks or just leave it as it is.
No, I think we should instead move the other trylocks to the try-for-ownership model as well, like Linus suggested.
That's the general assumption we make in locking primitives, that we optimize for the common, expected case - which would be that the trylock succeeds, and I don't see why trylock primitives should be different.
In fact I can see more ways for read-for-sharing to perform suboptimally on larger systems.
Thanks,
Ingo
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