Subject: Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()

From: Ingo Molnar <[email-masked]>
Date: Wed, 13 Feb 2019 08:45:44 +0100

* Waiman Long <[email-masked]> 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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