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Thu, 30 Oct 2025 03:43:33 -0700 (PDT) Received: from [192.168.10.48] ([151.95.110.222]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-429952de5f9sm31532680f8f.38.2025.10.30.03.43.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Oct 2025 03:43:33 -0700 (PDT) Message-ID: Date: Thu, 30 Oct 2025 11:43:30 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] rust: lock: Export Guard::do_unlocked() To: Lyude Paul , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, Benno Lossin Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Miguel Ojeda , Alex Gaynor , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich References: <20251029183538.226257-1-lyude@redhat.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/29/25 19:35, Lyude Paul wrote: > + /// // Since we hold work.lock, which work will also try to acquire in WorkItem::run. Dropping > + /// // the lock temporarily while we wait for completion works around this. > + /// g.do_unlocked(|| work.done.wait_for_completion()); > + /// > + /// assert_eq!(*g, 42); > + /// ``` > + pub fn do_unlocked(&mut self, cb: impl FnOnce() -> U) -> U { > // SAFETY: The caller owns the lock, so it is safe to unlock it. > unsafe { B::unlock(self.lock.state.get(), &self.state) }; Getting self as &mut is incorrect. That's because owning a lock guard implicitly tells you that no other thread can observe the intermediate states of the object. (The same is even more obviously true for a RefCell's mutable borrow, i.e. core::cell::RefMut) Let's say you have a lock-protected data structure with an invariant that is preserved at the end of every critical section. Let's say also that you have a function fn do_something() { let g = self.inner.lock(); g.mess_up_the_invariant(); // (1) self.do_something_else(&mut g); // uses do_unlocked() g.fix_the_invariant(); // (2) } Because the function holds a guard between the calls (1) and (2), it expects that other thread cannot observe the temporary state. The fact that do_unlocked() takes a &mut doesn't help, because the common case for RAII objects is that they're passed around mutably. Instead, do_unlocked should take the guard and return another one: fn do_something() { let mut g = self.inner.lock(); g.mess_up_the_invariant(); // (1) g = self.do_something_else(g); // uses do_unlocked() g.fix_the_invariant(); // (2) } This version of the interface makes it clear that (1) and (2) are in a separate critical section. Unfortunately it makes the signature uglier for do_unlocked() itself: #[must_use] pub fn do_unlocked(self, cb: impl FnOnce() -> U) -> (Self, U) Paolo