linux-kernel - Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

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Date:   Tue, 4 Jun 2019 14:04:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning

On 6/4/19 1:38 PM, Peter Zijlstra wrote:
> On Tue, Jun 04, 2019 at 01:30:00PM -0400, Waiman Long wrote:
>>> That's somewhat inconsistent wrt the type. I'll make it unsigned long,
>>> as that is what makes most sense, given there's a pointer inside.
>> Thank for spotting that, I will fix it.
> I fixed a whole bunch of them; please find the modified patches here:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=locking/core

Thanks for reviewing the patches.

So how do you think about the overall state of this patchset? Do you
think it is mature enough to go into 5.3?

Or if you want more time to think about solving the RT thread issue, we
can merge just patches 1-16 and play with the last threes for some more
time. I am fine with that too as improving RT tasks is not my main
focus. I like patch 16 as it led me to discover the rwsem reader wakeup
bug as I hit the negative dentry count WARN_ON message in my testing.

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.

Cheers,
Longman