linux-kernel - Re: [PATCH 12/13] rust: sync: introduce `CondVar`

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Message-ID: <20230330125223.GC124812@hirez.programming.kicks-ass.net>
Date:   Thu, 30 Mar 2023 14:52:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 12/13] rust: sync: introduce `CondVar`

On Thu, Mar 30, 2023 at 01:39:53AM -0300, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@...rosoft.com>
> 
> This is the traditional condition variable or monitor synchronisation
> primitive. It is implemented with C's `wait_queue_head_t`.
> 
> It allows users to release a lock and go to sleep while guaranteeing
> that notifications won't be missed. This is achieved by enqueuing a wait
> entry before releasing the lock.
> 

> +/// A conditional variable.
> +///
> +/// Exposes the kernel's [`struct wait_queue_head`] as a condition variable. It allows the caller to
> +/// atomically release the given lock and go to sleep. It reacquires the lock when it wakes up. And
> +/// it wakes up when notified by another thread (via [`CondVar::notify_one`] or
> +/// [`CondVar::notify_all`]) or because the thread received a signal. It may also wake up
> +/// spuriously.

Urgh so wide :-/

But no, threads can *always* and for any reason, have spurious wakeups.

Also, is this hard tied to mutex? If so, you should probably use swait
instead of wait.