linux-kernel - Re: [RFC PATCH] locking/rwsem: Add upgrade

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Message-ID: <000af44f-6f24-5a2d-8700-452469b3d6b3@redhat.com>
Date:   Wed, 22 Sep 2021 16:19:31 -0400
From:   Waiman Long <llong@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RFC PATCH] locking/rwsem: Add upgrade_read()

On 9/22/21 4:06 PM, Peter Zijlstra wrote:
> On Wed, Sep 22, 2021 at 03:36:57PM -0400, Waiman Long wrote:
>> Currently there are about 12 instances in the kernel where an up_read()
>> is immediately followed by a down_write() of the same lock. For example,
>>
>>    drivers/tty/n_tty.c:		up_read(&tty->termios_rwsem);
>>    drivers/tty/n_tty.c-		down_write(&tty->termios_rwsem);
> And TTY is a high performance issue, that requires hacks like this?
I won't call it a hack. I consider it a better documentation what the 
real intention is.
>
>> Since we have already provided a downgrade_write() function, we may as
>> well provide an upgrade_read() function to make the code easier to read
>> and the intention clearer.
>>
>> If the current task is the only reader, the upgrade can be done by a
>> single atomic operation. If not, the upgrade will have to be done by a
>> separate up_read() call followed by a down_write(). In the former case,
>> the handoff bit is not considered and the waiter will have to wait a
>> bit longer to acquire the lock.
>>
>> The new upgrade_read() function returns a value of 0 for safe upgrade
>> where rwsem protected data won't change. Otherwise a value of 1 is
>> returned to indicate unsafe upgrade where rwsem protected data may
>> change during the upgrade process.
> Yuck...
>
> Is there any workload where this is a massive win? I'm thinking that
> either the lock is contended and you get the unsafe option which is the
> same as today, or the lock isn't contended and you would've gotten
> fast-paths and you barely safe anything anyway.
>
> Also, -ENODATA
>
Yes, the best case saving is just just an atomic op.

This is just a RFC. I can look for workloads that can benefit from using 
this.

Cheers,
Longman