Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 4 Jun 2020 07:18:37 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, frederic@kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org, cai@lca.pw,
        mgorman@techsingularity.net
Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list
Message-ID: <20200604141837.GA179816@roeck-us.net>
References: <20200526161057.531933155@infradead.org>
 <20200526161908.129371594@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200526161908.129371594@infradead.org>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, May 26, 2020 at 06:11:04PM +0200, Peter Zijlstra wrote:
> The recent commit: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> got smp_call_function_single_async() subtly wrong. Even though it will
> return -EBUSY when trying to re-use a csd, that condition is not
> atomic and still requires external serialization.
> 
> The change in ttwu_queue_remote() got this wrong.
> 
> While on first reading ttwu_queue_remote() has an atomic test-and-set
> that appears to serialize the use, the matching 'release' is not in
> the right place to actually guarantee this serialization.
> 
> The actual race is vs the sched_ttwu_pending() call in the idle loop;
> that can run the wakeup-list without consuming the CSD.
> 
> Instead of trying to chain the lists, merge them.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
...
> +	/*
> +	 * Assert the CSD_TYPE_TTWU layout is similar enough
> +	 * for task_struct to be on the @call_single_queue.
> +	 */
> +	BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
> +		     offsetof(struct __call_single_data, flags) - offsetof(struct __call_single_data, llist));
> +

There is no guarantee in C that

	type1 a;
	type2 b;

in two different data structures means that offsetof(b) - offsetof(a)
is the same in both data structures unless attributes such as
__attribute__((__packed__)) are used.

As result, this does and will cause a variety of build errors depending
on the compiler version and compile flags.

Guenter