Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list

[Guenter Roeck <linux@roeck-us.net>]

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