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[Qemu-devel] [PATCH 5/8] timers: prepare the code for future races in ca
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 5/8] timers: prepare the code for future races in calling qemu_clock_warp |
Date: |
Tue, 8 Oct 2013 10:47:35 +0200 |
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;
}
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
[Qemu-devel] [PATCH 6/8] timers: introduce cpu_get_clock_locked, Paolo Bonzini, 2013/10/08
[Qemu-devel] [PATCH 7/8] timers: document (future) locking rules for icount, Paolo Bonzini, 2013/10/08
[Qemu-devel] [PATCH 8/8] timers: make icount thread-safe, Paolo Bonzini, 2013/10/08