On Tue, 14 Sept 2021 at 17:49, Linus Torvalds
[off-list ref] wrote:

On Tue, Sep 14, 2021 at 5:11 AM Ard Biesheuvel [off-list ref] wrote:

quoted

 static inline unsigned int task_cpu(const struct task_struct *p)
 {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
-       return READ_ONCE(p->cpu);
+       return READ_ONCE(p->thread_info.cpu);
 #else
        return READ_ONCE(task_thread_info(p)->cpu);
 #endif

Those two lines look different, but aren't.

Please just remove the CONFIG_THREAD_INFO_IN_TASK conditional, and use

          return READ_ONCE(task_thread_info(p)->cpu);

unconditionally, which now does the right thing regardless.

Unfortunately not.

task_cpu() takes a 'const struct task_struct *', whereas
task_thread_info() takes a 'struct task_struct *'.

Since task_thread_info()-><foo> is widely used as an lvalue, I would
need to update task_cpu()'s prototype and fix up all the callers, some
of which take the const flavor themselves. Or introduce
'const_task_thread_info()' which takes the const flavor, and cannot be
used to instantiate lvalues.

Suggestions welcome, but this is the cleanest I could come up with.