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icount: prepare the code for future races in calling qemu_clock_warp

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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 <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2013-10-07 17:30:02 +02:00
parent 8ed961d957
commit ce78d18ced
1 changed files with 10 additions and 12 deletions
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22
cpus.c
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@ -329,6 +329,7 @@ void qtest_clock_warp(int64_t dest)
void qemu_clock_warp(QEMUClockType type)
{
int64_t clock;
int64_t deadline;
/*
@ -348,8 +349,8 @@ 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;
}
@ -358,17 +359,11 @@ 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;
}
if (deadline > 0) {
@ -389,7 +384,10 @@ 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);
}