On Tue, 26 May 2020 18:11:04 +0200, Peter Zijlstra said:
> 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.

>  kernel/smp.c          |   47 ++++++++++++++++++++++++++++++++++++++++-------

> --- a/kernel/smp.c
> +++ b/kernel/smp.c

> @@ -659,6 +685,13 @@ void __init smp_init(void)
>  	BUILD_BUG_ON(offsetof(struct irq_work, flags) !=
>  		     offsetof(struct __call_single_data, flags));
>
> +	/*
> +	 * 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));
> +
>  	idle_threads_init();
>  	cpuhp_threads_init();

This blows up on a 32-bit ARM allmodconfig:

  CC      kernel/smp.o
kernel/smp.c:319:6: warning: no previous prototype for 'flush_smp_call_function_from_idle' [-Wmissing-prototypes]
 void flush_smp_call_function_from_idle(void)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./arch/arm/include/asm/atomic.h:11,
                 from ./include/linux/atomic.h:7,
                 from ./include/linux/llist.h:51,
                 from ./include/linux/irq_work.h:5,
                 from kernel/smp.c:10:
kernel/smp.c: In function 'smp_init':
./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_152' declared with attribute error: BUILD_BUG_ON failed: 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)
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
./include/linux/compiler.h:373:4: note: in definition of macro '__compiletime_assert'
    prefix ## suffix();    \
    ^~~~~~
./include/linux/compiler.h:392:2: note: in expansion of macro '_compiletime_assert'
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
  ^~~~~~~~~~~~~~~~
kernel/smp.c:689:2: note: in expansion of macro 'BUILD_BUG_ON'
  BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
  ^~~~~~~~~~~~