linux-kernel - Re: [PATCH] workqueue: rust: add creation of workqueues

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Message-ID: <CAH5fLghxUEzeCzjx_kLXvkZZWSLiGp-Uo28DQ-psrno5207XsQ@mail.gmail.com>
Date: Wed, 16 Apr 2025 22:18:13 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tejun Heo <tj@...nel.org>
Cc: Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Daniel Almeida <daniel.almeida@...labora.com>, 
	Tamir Duberstein <tamird@...il.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: rust: add creation of workqueues

On Wed, Apr 16, 2025 at 10:14 PM Tejun Heo <tj@...nel.org> wrote:
>
> On Wed, Apr 16, 2025 at 10:10:41AM -1000, Tejun Heo wrote:
> > On Wed, Apr 16, 2025 at 10:08:35PM +0200, Alice Ryhl wrote:
> > ...
> > > > This should be pretty cheap and we can probably enable this for everyone,
> > > > but if the overhead is noticeable, this can be an optional behavior
> > > > depending on a workqueue flag.
> > >
> > > My only concern is that we're executing work items *before* the
> > > deadline they specified. There could be work items that assume this
> > > doesn't happen? But maybe it's okay. Otherwise, what you suggest seems
> > > reasonable enough to me.
> >
> > That's already what flush_delayed_work() does, so I don't think it'd be too
> > surprising. Alternatively, we can go for canceling on draining/destruction
> > but that'd be more surprising I think. As long as the behavior is documented
> > clearly, I don't see problems with running and flushing them.
>
> Also, note that self-requeueing work items may still be pending after
> draining a workqueue as the draining is best effort. This is considered a
> bug in the caller and, we trigger a warn and just skip freeing the
> workqueue. This is again not great but may be acceptable for rust too. If
> one wants to improve this, now that we have disable_work(), we can probably
> trigger warn after X retries and then switch to disabling & flushing
> afterwards.

Rust doesn't let you call methods on a struct while its destructor is
running, so I don't think this bug is possible in Rust, since it
involves calling `enqueue` on a `Queue` whose destructor is currently
running.

Alice