Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
Date:   Fri, 08 Dec 2023 09:30:41 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] rust: sync: add `CondVar::notify_sync`
Message-ID: <2TyvgVxGiHf9tLVgLAuw9h9gjv5iM0f_5KE0bM4PGIWrnNXRASVZ-UwxnI4v1XQEIp6mkbTTg9Sr7jEigChMd5Sxhpnd2NV8IDJ9sDXgIPo=@proton.me>
In-Reply-To: <CAH5fLgi37LER-G-ozc13TWGGxraFmP3QVYqX7oBoahyKwgfznQ@mail.gmail.com>
References: <20231206-rb-new-condvar-methods-v1-0-33a4cab7fdaa@google.com> <20231206-rb-new-condvar-methods-v1-1-33a4cab7fdaa@google.com> <n18Cdl9DQXfuGtD6d664Mn0rrPfxebZs31-3vQfu5N0vP0ZgfHNwBZYC4qYgU1GD-4Ue4bUhgNqs40-u8t0baAzlQ0cMMiSzTgjQnhcG50o=@proton.me> <CAH5fLgi37LER-G-ozc13TWGGxraFmP3QVYqX7oBoahyKwgfznQ@mail.gmail.com>
Feedback-ID: 71780778:user:proton
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On 12/8/23 08:29, Alice Ryhl wrote:
> On Thu, Dec 7, 2023 at 9:22=E2=80=AFPM Benno Lossin <benno.lossin@proton.=
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@google.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(), bindin=
gs::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?
>=20
> No, we don't need to hold a lock. The call stack is:
>=20
> 1. __wake_up_sync
> 2. __wake_up_sync_key
> 3. __wake_up_common_lock
> 4. __wake_up_common
>=20
> 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@proton.me>

--=20
Cheers,
Benno