Re: [Qemu-devel] [PATCH 5/8] timers: prepare the code for future races in calling qemu_clock_warp

[Alex Bligh]

On 8 Oct 2013, at 09:47, Paolo Bonzini wrote:

> Computing the deadline of all vm_clocks is somewhat expensive and calls
> out to qemu-timer.c; two reasons not to do it in the seqlock's write-side
> critical section.  This however opens the door for races in setting and
> reading vm_clock_warp_start.
> 
> To plug them, we need to cover the case where a new deadline slips in
> between the call to qemu_clock_deadline_ns_all and the actual modification
> of the icount_warp_timer.  Restrict changes to vm_clock_warp_start and
> the icount_warp_timer's expiration time, to only move them back (which
> would simply cause an early wakeup).
> 
> If a vm_clock timer is cancelled while CPUs are idle, this might cause the
> icount_warp_timer to fire unnecessarily.  This is not a problem, after it
> fires the timer becomes inactive and the next call to timer_mod_anticipate
> will be precise.
> 
> In addition to this, we must deactivate the icount_warp_timer _before_
> checking whether CPUs are idle.  This way, if the "last" CPU becomes idle
> during the call to timer_del we will still set up the icount_warp_timer.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
> cpus.c | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index 9f450ad..08eaf23 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -319,6 +319,7 @@ void qtest_clock_warp(int64_t dest)
> 
> void qemu_clock_warp(QEMUClockType type)
> {
> +    int64_t clock;
>     int64_t deadline;
> 
>     /*
> @@ -338,7 +339,7 @@ void qemu_clock_warp(QEMUClockType type)
>      * the earliest QEMU_CLOCK_VIRTUAL timer.
>      */
>     icount_warp_rt(NULL);
> -    if (!all_cpu_threads_idle() || 
> !qemu_clock_has_timers(QEMU_CLOCK_VIRTUAL)) {
> -        timer_del(icount_warp_timer);
> +    timer_del(icount_warp_timer);
> +    if (!all_cpu_threads_idle()) {
>         return;
>     }
> @@ -348,17 +349,11 @@ void qemu_clock_warp(QEMUClockType type)
>       return;
>     }
> 
> -    vm_clock_warp_start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>     /* We want to use the earliest deadline from ALL vm_clocks */
> +    clock = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>     deadline = qemu_clock_deadline_ns_all(QEMU_CLOCK_VIRTUAL);
> -
> -    /* Maintain prior (possibly buggy) behaviour where if no deadline
> -     * was set (as there is no QEMU_CLOCK_VIRTUAL timer) or it is more than
> -     * INT32_MAX nanoseconds ahead, we still use INT32_MAX
> -     * nanoseconds.
> -     */
> -    if ((deadline < 0) || (deadline > INT32_MAX)) {
> -        deadline = INT32_MAX;
> +    if (deadline < 0) {
> +        return;
>     }

Arguably the patch could document why removing the check for deadline > 
INT32_MAX
(the bug for bug compatibility) is safe, as I couldn't entirely convince myself 
it
was, mostly because I couldn't see why it was doing it in the first place.

> 
>     if (deadline > 0) {
> @@ -379,7 +375,10 @@ void qemu_clock_warp(QEMUClockType type)
>          * you will not be sending network packets continuously instead of
>          * every 100ms.
>          */
> -        timer_mod(icount_warp_timer, vm_clock_warp_start + deadline);
> +        if (vm_clock_warp_start == -1 || vm_clock_warp_start > clock) {
> +            vm_clock_warp_start = clock;
> +        }
> +        timer_mod_anticipate(icount_warp_timer, clock + deadline);
>     } else if (deadline == 0) {
>         qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
>     }
> -- 
> 1.8.3.1
> 
> 
> 
> 

-- 
Alex Bligh