linux-kernel - Re: [PATCH 1/2] rust: sync: add `CondVar::notify

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Message-ID: <2TyvgVxGiHf9tLVgLAuw9h9gjv5iM0f_5KE0bM4PGIWrnNXRASVZ-UwxnI4v1XQEIp6mkbTTg9Sr7jEigChMd5Sxhpnd2NV8IDJ9sDXgIPo=@proton.me>
Date:   Fri, 08 Dec 2023 09:30:41 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: sync: add `CondVar::notify_sync`

On 12/8/23 08:29, Alice Ryhl wrote:
> On Thu, Dec 7, 2023 at 9:22 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> On 12/6/23 11:09, Alice Ryhl wrote:
>>> Wake up another thread synchronously.
>>>
>>> This method behaves like `notify_one`, except that it hints to the
>>> scheduler that the current thread is about to go to sleep, so it should
>>> schedule the target thread on the same CPU.
>>>
>>> This is used by Rust Binder as a performance optimization. When sending
>>> a transaction to a different process, we usually know which thread will
>>> handle it, so we can schedule that thread for execution next on this
>>> CPU for better cache locality.
>>>
>>> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>>> ---
>>>  rust/kernel/sync/condvar.rs | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
>>> index b679b6f6dbeb..9861c6749ad0 100644
>>> --- a/rust/kernel/sync/condvar.rs
>>> +++ b/rust/kernel/sync/condvar.rs
>>> @@ -155,6 +155,12 @@ fn notify(&self, count: i32, flags: u32) {
>>>          };
>>>      }
>>>
>>> +    /// Calls the kernel function to notify one thread synchronously.
>>> +    pub fn notify_sync(&self) {
>>> +        // SAFETY: `wait_list` points to valid memory.
>>> +        unsafe { bindings::__wake_up_sync(self.wait_list.get(), bindings::TASK_NORMAL) };
>>
>> I took a look at the C function (i.e. __wake_up_common) and there I
>> found this:
>>
>>     lockdep_assert_held(&wq_head->lock);
>>
>> So I think this function requires that the lock is held, how are you
>> ensuring this?
> 
> No, we don't need to hold a lock. The call stack is:
> 
> 1. __wake_up_sync
> 2. __wake_up_sync_key
> 3. __wake_up_common_lock
> 4. __wake_up_common
> 
> And __wake_up_common_lock will lock wq_head->lock before calling
> __wake_up_common.

Seems like I just looked at the wrong function.

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

-- 
Cheers,
Benno