From: "André Almeida" <andrealmeid@igalia.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
"Paul E . McKenney" <paulmck@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
"H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
linux-api@vger.kernel.org, Florian Weimer <fw@deneb.enyo.de>,
David.Laight@aculab.com, carlos@redhat.com,
Peter Oskolkov <posk@posk.io>,
Alexander Mikhalitsyn <alexander@mihalicyn.com>,
Chris Kennelly <ckennelly@google.com>,
Ingo Molnar <mingo@redhat.com>,
Darren Hart <dvhart@infradead.org>,
Davidlohr Bueso <dave@stgolabs.net>,
libc-alpha@sourceware.org, Steven Rostedt <rostedt@goodmis.org>,
Jonathan Corbet <corbet@lwn.net>,
Noah Goldstein <goldstein.w.n@gmail.com>,
Daniel Colascione <dancol@google.com>,
longman@redhat.com, kernel-dev@igalia.com
Subject: Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation
Date: Wed, 1 May 2024 20:44:36 -0300 [thread overview]
Message-ID: <f052ff72-72c9-4b83-9285-2cd9d52e5f72@igalia.com> (raw)
In-Reply-To: <20240426-gaumen-zweibeinig-3490b06e86c2@brauner>
Hi Christian,
Em 26/04/2024 07:26, Christian Brauner escreveu:
> On Thu, Apr 25, 2024 at 05:43:31PM -0300, André Almeida wrote:
>> Hi,
>>
>> In the last LPC, Mathieu Desnoyers and I presented[0] a proposal to extend the
>> rseq interface to be able to implement spin locks in userspace correctly. Thomas
>> Gleixner agreed that this is something that Linux could improve, but asked for
>> an alternative proposal first: a futex operation that allows to spin a user
>> lock inside the kernel. This patchset implements a prototype of this idea for
>> further discussion.
>>
>> With FUTEX2_SPIN flag set during a futex_wait(), the futex value is expected to
>> be the PID of the lock owner. Then, the kernel gets the task_struct of the
>> corresponding PID, and checks if it's running. It spins until the futex
>> is awaken, the task is scheduled out or if a timeout happens. If the lock owner
>> is scheduled out at any time, then the syscall follows the normal path of
>> sleeping as usual.
>>
>> If the futex is awaken and we are spinning, we can return to userspace quickly,
>> avoid the scheduling out and in again to wake from a futex_wait(), thus
>> speeding up the wait operation.
>>
>> I didn't manage to find a good mechanism to prevent race conditions between
>> setting *futex = PID in userspace and doing find_get_task_by_vpid(PID) in kernel
>> space, giving that there's enough room for the original PID owner exit and such
>> PID to be relocated to another unrelated task in the system. I didn't performed
>
> One option would be to also allow pidfds. Starting with v6.9 they can be
> used to reference individual threads.
>
> So for the really fast case where you have multiple threads and you
> somehow may really do care about the impact of the atomic_long_inc() on
> pidfd_file->f_count during fdget() (for the single-threaded case the
> increment is elided), callers can pass the TID. But in cases where the
> inc and put aren't a performance sensitive, you can use pidfds.
>
Thank you very much for making the effort here, much appreciated :)
While I agree that pidfds would fix the PID race conditions, I will move
this interface to support TIDs instead, as noted by Florian and Peter.
With TID the race conditions are diminished I reckon?
next prev parent reply other threads:[~2024-05-01 23:45 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-25 20:43 [RFC PATCH 0/1] Add FUTEX_SPIN operation André Almeida
2024-04-25 20:43 ` [RFC PATCH 1/1] futex: " André Almeida
2024-04-26 9:43 ` [RFC PATCH 0/1] " Florian Weimer
2024-04-26 10:14 ` Peter Zijlstra
2024-04-26 10:26 ` Christian Brauner
2024-05-01 23:44 ` André Almeida [this message]
2024-05-02 8:45 ` Christian Brauner
2024-05-02 9:51 ` Florian Weimer
2024-05-02 10:14 ` Christian Brauner
2024-05-02 10:39 ` Florian Weimer
2024-05-02 13:08 ` Christian Brauner
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