linux-kernel - Re: [PATCH] rust: sync: document that Guard is not a stable lock guard

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Message-ID: <CAH5fLgi8N6bdke1o_uTvYsYfwH1KhsY4wTGmFpc43Mi_U6+KWg@mail.gmail.com>
Date: Fri, 6 Dec 2024 10:56:23 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...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>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: sync: document that Guard is not a stable lock guard

On Thu, Dec 5, 2024 at 7:18 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Thu, Dec 05, 2024 at 12:35:51PM +0000, Alice Ryhl wrote:
> > Most locks in the linux kernel are stable, which means that holding the
> > lock is sufficient to keep the value from being freed. For example, this
> > means that if you acquire a lock on a refcounted value during rcu, then
> > you do not need to acquire a refcount to keep it alive past
> > rcu_read_unlock().
> >
> > However, the Rust `Guard` type is written in a way where it cannot be
> > used with this pattern. One reason for this is the existence of the
> > `do_unlocked` method that is used with `Condvar`. The method allows you
> > to unlock the lock, run some code, and then reacquire the lock. This
> > operation is not okay if the lock itself is what keeps the value alive,
> > as it could be freed right after the unlock call.
> >
>
> Hmm... but `Guard` holds a reference to the corresponding `Lock`. How
> could this happen? Do you have an example?

Well it can't. The reference is yet another reason that Guard can't be
used for stable locking.

This doc change arises out of me needing a stable lock for something.

Alice