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Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning
From:   Waiman Long <longman@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
References: <20190520205918.22251-1-longman@redhat.com>
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 <8e7d19ea-f2e6-f441-6ab9-cbff6d96589c@redhat.com>
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Organization: Red Hat
Message-ID: <46e44f43-87fd-251b-3b83-89a8bb3b407f@redhat.com>
Date:   Wed, 5 Jun 2019 14:13:27 -0400
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On 6/4/19 2:21 PM, Waiman Long wrote:
> On 6/4/19 2:14 PM, Peter Zijlstra wrote:
>>> I worked on this owner merging patch mainly to alleviate the need to use
>>> cmpxchg for reader lock. cmpxchg_double() is certainly one possible
>>> solution though it won't work on older CPUs. We can have a config option
>>> to use cmpxchg_double as it may increase the size of other structures
>>> that embedded rwsem and impose additional alignment constraint.
>> cmpxchg8b was introduced with the Pentium (for PAE IIRC, it enabled
>> atomic 64bit PTEs, but Linux never used it for that) and every Intel/AMD
>> thereafter has had it. AFAIK there's no x86_64 chip without cmpxchg16b.
> Thank for the clarification. I actually didn't check when cmpxch8b was
> introduced. I know it is a bit slower than regular cmpxchg. So we may
> still need to do some performance analysis to see how it compares with
> my current approach.

Using cmpxchg_double is actually more risky than I thought. I have been
trying to try to use cmpxchg_double for down_write, but I kept getting
kernel panics because the rwsem wasn't 16b-aligned. As rwsem is embedded
in quite a large number of structures, they all have to align properly
to make that work or the kernel will panic. That does seem too risky to
me. So I am dropping the idea of trying to use it.

Cheers,
Longman