Re: [PATCH v2 2/2] x86/fpu: Update the debug flow for x86_task_fpu()

[Sohil Mehta <sohil.mehta@intel.com>]

On 7/21/2025 3:34 PM, Dave Hansen wrote:
> On 7/21/25 14:53, Sohil Mehta wrote:
>> Also, update the warning to include PF_USER_WORKER, as these tasks are
>> treated the same as kernel threads in the FPU context.
> 
> How so?

There are quite a few checks that special case PF_KTHREAD along with
PF_USER_WORKER. For example, FPSTATE allocation and freeing as well as
some context switch paths. Did I misinterpret something?

I believe it originates here:
https://lore.kernel.org/all/560c844c-f128-555b-40c6-31baff27537f@kernel.dk/

switch_fpu():
	if (!test_tsk_thread_flag(old, TIF_NEED_FPU_LOAD) &&
	    cpu_feature_enabled(X86_FEATURE_FPU) &&
	    !(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
		struct fpu *old_fpu = x86_task_fpu(old);


arch_release_task_struct():
	if (fpu_state_size_dynamic() && !(tsk->flags & (PF_KTHREAD |
PF_USER_WORKER)))
		fpstate_free(x86_task_fpu(tsk));


fpregs_restore_userregs():
if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER)))
	return;

kernel_fpu_begin_mask():
	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
		set_thread_flag(TIF_NEED_FPU_LOAD);
		save_fpregs_to_fpstate(x86_task_fpu(current));
	}