linux-kernel - Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

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Message-ID: <20210216102044.e2cvvqdglqs23yfe@linutronix.de>
Date:   Tue, 16 Feb 2021 11:20:44 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     André Almeida <andrealmeid@...labora.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        kernel@...labora.com, krisman@...labora.com,
        pgriffais@...vesoftware.com, z.figura12@...il.com,
        joel@...lfernandes.org, malteskarupke@...tmail.fm,
        linux-api@...r.kernel.org, fweimer@...hat.com,
        libc-alpha@...rceware.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, acme@...nel.org, corbet@....net
Subject: Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

On 2021-02-16 10:56:14 [+0100], Peter Zijlstra wrote:
> So while I'm in favour of adding a new interface, I'm not sure I see
> benefit of reimplementing the basics, sure it seems simpler now, but
> that's because you've not implemented all the 'fun' stuff.

The last attempt tried to hide the updated interface within libc which
did not fly. The global hash state is one of the problems because it
leads to hash collisions of two unrelated locks.
It will get simpler if we go into the kernel for each lock/unlock
operation but this might not very good in terms of performance for locks
which are mostly uncontended. I'm not sure how much we can cheat in
terms of VDSO.

Sebastian