[PATCH v2 1/2] um: time-travel: fix IRQ handling in time_travel_handle_message()

[Anton Ivanov]

On 10/09/2020 10:31, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg at intel.com>
> 
> As the comment here indicates, we need to do the polling in the
> idle loop without blocking interrupts, since interrupts can be
> vhost-user messages that we must process even while in our idle
> loop.
> 
> I don't know why I explained one thing and implemented another,
> but we have indeed observed random hangs due to this, depending
> on the timing of the messages.
> 
> Fixes: 88ce64249233 ("um: Implement time-travel=ext")
> Signed-off-by: Johannes Berg <johannes.berg at intel.com>
> ---
>   arch/um/kernel/time.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
> index 25eaa6a0c658..c07436e89e59 100644
> --- a/arch/um/kernel/time.c
> +++ b/arch/um/kernel/time.c
> @@ -70,13 +70,17 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
>   	 * read of the message and write of the ACK.
>   	 */
>   	if (mode != TTMH_READ) {
> +		bool disabled = irqs_disabled();
> +
> +		BUG_ON(mode == TTMH_IDLE && !disabled);
> +
> +		if (disabled)
> +			local_irq_enable();
>   		while (os_poll(1, &time_travel_ext_fd) != 0) {
> -			if (mode == TTMH_IDLE) {
> -				BUG_ON(!irqs_disabled());
> -				local_irq_enable();
> -				local_irq_disable();
> -			}
> +			/* nothing */
>   		}
> +		if (disabled)
> +			local_irq_disable();
>   	}
>   
>   	ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));
> 

Acked-By: Anton Ivanov <anton.ivanov at cambridgegreys.com>

-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/