[PATCH] powerpc/pseries/vas: Migration suspend waits for no in-progress open windows

Nathan Lynch nathanl at linux.ibm.com
Tue Oct 10 07:09:26 AEDT 2023

Hi Haren,

Haren Myneni <haren at linux.ibm.com> writes:
> The hypervisor returns migration failure if all VAS windows are not
> closed. During pre-migration stage, vas_migration_handler() sets
> migration_in_progress flag and closes all windows from the list.
> The allocate VAS window routine checks the migration flag, setup
> the window and then add it to the list. So there is possibility of
> the migration handler missing the window that is still in the
> process of setup.
> t1: Allocate and open VAS	t2: Migration event
>     window
> lock vas_pseries_mutex
> If migration_in_progress set
>   unlock vas_pseries_mutex
>   return
> open window HCALL
> unlock vas_pseries_mutex
> Modify window HCALL		lock vas_pseries_mutex
> setup window			migration_in_progress=true
> 				Closes all windows from
> 				the list
> 				unlock vas_pseries_mutex
> lock vas_pseries_mutex		return
> if nr_closed_windows == 0
>   // No DLPAR CPU or migration
>   add to the list
>   unlock vas_pseries_mutex
>   return
> unlock vas_pseries_mutex
> Close VAS window
> // due to DLPAR CPU or migration
> return -EBUSY

Could the the path t1 takes simply hold the mutex for the duration of
its execution instead of dropping and reacquiring it in the middle?

Here's the relevant code from vas_allocate_window():

	if (migration_in_progress)
		rc = -EBUSY;
		rc = allocate_setup_window(txwin, (u64 *)&domain[0],
	if (rc)
		goto out;

	rc = h_modify_vas_window(txwin);
	if (!rc)
		rc = get_vas_user_win_ref(&txwin->vas_win.task_ref);
	if (rc)
		goto out_free;

	txwin->win_type = cop_feat_caps->win_type;
	if (!caps->nr_close_wins) {
		list_add(&txwin->win_list, &caps->list);
		return &txwin->vas_win;

Is there something about h_modify_vas_window() or get_vas_user_win_ref()
that requires temporarily dropping the lock?

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