Re: [PATCH v2] smp: align struct __call_single_node to 8 bytes

[David Laight]

On Thu, 24 Sep 2026 11:34:50 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Wed, Sep 23, 2026 at 08:43:07AM -0700, Brian Cain wrote:
> > Consumers walking call_single_queue cast each node to call_single_data_t
> > and, given that type's declared over-alignment, the compiler may load
> > llist.next and u_flags as one naturally aligned double word.  Fields using
> > this type, such as task_struct::wake_entry only guarantee pointer alignment,
> > which faults on strict-alignment 32-bit architectures like hexagon.  
> 
> Where exactly does this happen? I'm failing with grep.
> 

Here for one:

void __smp_call_single_queue(int cpu, struct llist_node *node)
{
	/*
	 * We have to check the type of the CSD before queueing it, because
	 * once queued it can have its flags cleared by
	 *   flush_smp_call_function_queue()
	 * even if we haven't sent the smp_call IPI yet (e.g. the stopper
	 * executes migration_cpu_stop() on the remote CPU).
	 */
	if (trace_csd_queue_cpu_enabled()) {
		call_single_data_t *csd;
		smp_call_func_t func;

		csd = container_of(node, call_single_data_t, node.llist);
		func = CSD_TYPE(csd) == CSD_TYPE_TTWU ?
			sched_ttwu_pending : csd->func;

		trace_call__csd_queue_cpu(cpu, _RET_IP_, func, csd);
	}

since the data item itself was probably created with:

#define CSD_INIT(_func, _info) \
	(struct __call_single_data){ .func = (_func), .info = (_info), }

I've not looked hard enough to see what the extra alignment was trying
to achieve.
But the above function needs to use the 'less aligned' struct __call_single_data
even if some places use call_single_data_t to get a 'more aligned' structure.

David