Hi Paolo,
please check the new version. I removed changing the iothread_locked
variable. But I still need to set the correct value of iothread_locked
when using qemu_cond_wait. This will fix my race condition on Windows
when prepare_mmio_access is called and checks if the lock is already
held by the current thread.
David
Commit afbe70535ff1a8a7a32910cc15ebecc0ba92e7da introduced the function
qemu_mutex_iothread_locked to avoid recursive locking of the iothread
lock. But the function qemu_cond_wait directly acquires the lock without
modifying iothread_locked. This leads to condition where
qemu_tcg_wait_io_event acquires the mutex by using qemu_cond_wait and
later calls tcg_exec_all > tcg_cpu_exec -> cpu_exec >
prepare_mmio_access checking if the current thread already holds the
lock. This is done by checking the variable iothread_locked which is
still 0, thus the function tries to acquire the mutex again.
The patch adds a function called qemu_cond_wait_iothread to keep
iothread_locked in sync.
Signed-off-by: David Engraf <david.eng...@sysgo.com>
---
cpus.c | 27 ++++++++++++++++++---------
1 file changed, 18 insertions(+), 9 deletions(-)
diff --git a/cpus.c b/cpus.c
index 877bd70..e4d2d4b 100644
--- a/cpus.c
+++ b/cpus.c
@@ -70,6 +70,8 @@ static CPUState *next_cpu;
int64_t max_delay;
int64_t max_advance;
+static void qemu_cond_wait_iothread(QemuCond *cond);
+
/* vcpu throttling controls */
static QEMUTimer *throttle_timer;
static unsigned int throttle_percentage;
@@ -920,7 +922,7 @@ void run_on_cpu(CPUState *cpu, void (*func)(void
*data), void *data)
while (!atomic_mb_read(&wi.done)) {
CPUState *self_cpu = current_cpu;
- qemu_cond_wait(&qemu_work_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_work_cond);
current_cpu = self_cpu;
}
}
@@ -998,11 +1000,11 @@ static void qemu_tcg_wait_io_event(CPUState *cpu)
/* Start accounting real time to the virtual clock if the CPUs
are idle. */
qemu_clock_warp(QEMU_CLOCK_VIRTUAL);
- qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(cpu->halt_cond);
}
while (iothread_requesting_mutex) {
- qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_io_proceeded_cond);
}
CPU_FOREACH(cpu) {
@@ -1013,7 +1015,7 @@ static void qemu_tcg_wait_io_event(CPUState *cpu)
static void qemu_kvm_wait_io_event(CPUState *cpu)
{
while (cpu_thread_is_idle(cpu)) {
- qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(cpu->halt_cond);
}
qemu_kvm_eat_signals(cpu);
@@ -1123,7 +1125,7 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
/* wait for initial kick-off after machine start */
while (first_cpu->stopped) {
- qemu_cond_wait(first_cpu->halt_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(first_cpu->halt_cond);
/* process any pending work */
CPU_FOREACH(cpu) {
@@ -1215,6 +1217,13 @@ bool qemu_mutex_iothread_locked(void)
return iothread_locked;
}
+static void qemu_cond_wait_iothread(QemuCond *cond)
+{
+ iothread_locked = false;
+ qemu_cond_wait(cond, &qemu_global_mutex);
+ iothread_locked = true;
+}
+
void qemu_mutex_lock_iothread(void)
{
atomic_inc(&iothread_requesting_mutex);
@@ -1277,7 +1286,7 @@ void pause_all_vcpus(void)
}
while (!all_vcpus_paused()) {
- qemu_cond_wait(&qemu_pause_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_pause_cond);
CPU_FOREACH(cpu) {
qemu_cpu_kick(cpu);
}
@@ -1326,7 +1335,7 @@ static void qemu_tcg_init_vcpu(CPUState *cpu)
cpu->hThread = qemu_thread_get_handle(cpu->thread);
#endif
while (!cpu->created) {
- qemu_cond_wait(&qemu_cpu_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_cpu_cond);
}
tcg_cpu_thread = cpu->thread;
} else {
@@ -1347,7 +1356,7 @@ static void qemu_kvm_start_vcpu(CPUState *cpu)
qemu_thread_create(cpu->thread, thread_name, qemu_kvm_cpu_thread_fn,
cpu, QEMU_THREAD_JOINABLE);
while (!cpu->created) {
- qemu_cond_wait(&qemu_cpu_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_cpu_cond);
}
}
@@ -1363,7 +1372,7 @@ static void qemu_dummy_start_vcpu(CPUState *cpu)
qemu_thread_create(cpu->thread, thread_name,
qemu_dummy_cpu_thread_fn, cpu,
QEMU_THREAD_JOINABLE);
while (!cpu->created) {
- qemu_cond_wait(&qemu_cpu_cond, &qemu_global_mutex);
+ qemu_cond_wait_iothread(&qemu_cpu_cond);
}
}
--
1.9.1
Am 25.11.2015 um 10:08 schrieb David Engraf:
Hi Paolo,
Am 24.11.2015 um 16:54 schrieb Paolo Bonzini:
On 24/11/2015 16:43, David Engraf wrote:
Commit afbe70535ff1a8a7a32910cc15ebecc0ba92e7da introduced the function
qemu_mutex_iothread_locked to avoid recursive locking of the iothread
lock. The iothread_locked variable uses the __thread attribute which
results in a per thread storage location whereas the qemu_global_mutex
is not thread specific. This leads to race conditions because the
variable is not in sync between threads.
Frankly, this makes no sense. You're modifying the
qemu_mutex_iothread_locked function to return whether _some_ thread has
taken the mutex, but the function tells you whether _the calling_ thread
has taken the mutex (that's the point about recursive locking). This
breaks the callers of qemu_mutex_iothread_locked completely.
The variable need not be in sync between the different threads; each
thread only needs to know about itself. The current code works because:
>
1) the iothread mutex is not locked before qemu_mutex_lock_iothread
2) the iothread mutex is not locked after qemu_mutex_lock_iothread
3) qemu_cond_wait doesn't matter because the thread does not run during
a qemu_cond_wait.
But this is my issue on Windows:
qemu_tcg_cpu_thread_fn
-> qemu_tcg_wait_io_event
-> qemu_cond_wait acquires the mutex
qemu_tcg_cpu_thread_fn
-> tcg_exec_all -> tcg_cpu_exec -> cpu_exec
-> cpu_exec ends up in calling prepare_mmio_access
prepare_mmio_access uses qemu_mutex_iothread_locked to check if
the lock is already held for this thread, but it returns 0
because qemu_cond_wait doesn't set iothread_locked but the mutex
is locked. Thus the function calls qemu_mutex_lock_iothread which
tries to acquire the mutex again. On Windows this results in an
assertion:
Assertion failed: mutex->owner == 0, file util/qemu-thread-win32.c, line 60
I'm using a self compiled, cross gcc-5.2.0 mingw compiler.
David
--
Mit freundlichen Grüßen/Best regards,
David Engraf
Product Engineer
SYSGO AG
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