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

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Message-ID: <Z1NCQiGRopuS-Uc8@tardis.local>
Date: Fri, 6 Dec 2024 10:28:18 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.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 Fri, Dec 06, 2024 at 10:56:23AM +0100, Alice Ryhl wrote:
> 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.
> 

Maybe it's better to put together this patch and the stable locking you
are working on? It's better for reviewing in that way.

I can see what a "stable lock' means, but want to make sure we change
the doc to reflect the exact requirement of a stable lock.

Regards,
Boqun

> Alice