Re: [PATCH V7 08/14] powerpc/vas: Take reference to PID and mm for user space windows

[Michael Ellerman <mpe@ellerman.id.au>]

Haren Myneni <haren@linux.ibm.com> writes:
> Process close windows after its requests are completed. In multi-thread
> applications, child can open a window but release FD will not be called
> upon its exit. Parent thread will be closing it later upon its exit.

What if the parent exits first?

> The parent can also send NX requests with this window and NX can
> generate page faults. After kernel handles the page fault, send
> signal to process by using PID if CSB address is invalid. Parent
> thread will not receive signal since its PID is different from the one
> saved in vas_window. So use tgid in case if the task for the pid saved
> in window is not running and send signal to its parent.
>
> To prevent reusing the pid until the window closed, take reference to
> pid and task mm.

That text is all very dense. Can you please flesh it out and reword it
to clearly spell out what's going on in much more detail.


> diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
> index a45d81d..7587258 100644
> --- a/arch/powerpc/platforms/powernv/vas-window.c
> +++ b/arch/powerpc/platforms/powernv/vas-window.c
> @@ -1266,8 +1300,17 @@ int vas_win_close(struct vas_window *window)
>  	poll_window_castout(window);
>  
>  	/* if send window, drop reference to matching receive window */
> -	if (window->tx_win)
> +	if (window->tx_win) {
> +		if (window->user_win) {
> +			/* Drop references to pid and mm */
> +			put_pid(window->pid);
> +			if (window->mm) {
> +				mmdrop(window->mm);
> +				mm_context_remove_copro(window->mm);

That seems backward. Once you drop the reference the mm can be freed
can't it?

> +			}
> +		}
>  		put_rx_win(window->rxwin);
> +	}
>  
>  	vas_window_free(window);

cheers