Re: [Qemu-devel] Re: [PATCH] Fix alarm_timer race with select

[Anthony Liguori <anthony@codemonkey.ws>]

Jan Kiszka wrote:
> Anthony Liguori wrote:
>   
>> Anthony Liguori wrote:
>>     
>>> Jan Kiszka wrote:
>>>
>>> Perhaps we should move the alarm timer check rearming out of the main
>>> loop and into a qemu_set_fd_handler2() handler?
>>>       
>> And now that I think about it, I see no reason why the timer expiration
>> checks couldn't be moved to the same handler.  I'd have to look more
>> closely at how that would interact with icount though.
>>     
>
> I cannot yet follow you here. But I my impression is that this will be
> an additional change that should come in a separate patch, no?
>
> Find the O_NONBLOCK remark addressed below.
>
> Jan
>
> ------------>
>
> Changing the default IO timeout to 5 s (#5578) made a race visible
> between the alarm_timer and select() in main_loop_wait(): If the timer
> fired before select was able to block, the full select() timeout could
> have been applied instead of returning immediately. Since #5578, this
> causes heavy problems to the Musicpal board emulation with stalls up to
> 5 s.
>
> The following patch introduces a pipe that is written to by
> host_alarm_handler and select()'ed in main_loop_wait(). This avoids
> prevents that select() blocks though a timer has fired and waits for
> processing.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
> ---
>  vl.c |   21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>
> Index: b/vl.c
> ===================================================================
> --- a/vl.c
> +++ b/vl.c
> @@ -884,6 +884,7 @@ static void qemu_rearm_alarm_timer(struc
>  #define MIN_TIMER_REARM_US 250
>  
>  static struct qemu_alarm_timer *alarm_timer;
> +static int alarm_timer_rfd, alarm_timer_wfd;
>  
>  #ifdef _WIN32
>  
> @@ -1303,12 +1304,15 @@ static void host_alarm_handler(int host_
>                                 qemu_get_clock(vm_clock))) ||
>          qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
>                             qemu_get_clock(rt_clock))) {
> +        CPUState *env = next_cpu;
> +        char byte = 0;
> +
>  #ifdef _WIN32
>          struct qemu_alarm_win32 *data = ((struct qemu_alarm_timer*)dwUser)->priv;
>          SetEvent(data->host_alarm);
>  #endif
> -        CPUState *env = next_cpu;
>  
> +        write(alarm_timer_wfd, &byte, sizeof(byte));
>          alarm_timer->flags |= ALARM_FLAG_EXPIRED;
>  
>          if (env) {
> @@ -1673,6 +1677,15 @@ static void init_timer_alarm(void)
>  {
>      struct qemu_alarm_timer *t = NULL;
>      int i, err = -1;
> +    int fds[2];
> +
> +    if (pipe(fds) || fcntl(fds[0], F_SETFL, O_NONBLOCK)
> +        || fcntl(fds[1], F_SETFL, O_NONBLOCK)) {
> +        perror("creating timer pipe");
> +        exit(1);
> +    }
> +    alarm_timer_rfd = fds[0];
> +    alarm_timer_wfd = fds[1];
>  
>      for (i = 0; alarm_timers[i].name; i++) {
>          t = &alarm_timers[i];
> @@ -4427,6 +4440,7 @@ void main_loop_wait(int timeout)
>      /* XXX: separate device handlers from system ones */
>      nfds = -1;
>      FD_ZERO(&rfds);
> +    FD_SET(alarm_timer_rfd, &rfds);
>   

Sorry, I missed this in the first patch.  You have to take 
alarm_timer_rfd into account when computing max_fd otherwise badness could
result.  While you're at it, please don't call pipe and fcntl() twice 
from inside of an if() clause.

Regards,

Anthony Liguori