[Qemu-devel] [PATCH] cpus: run work items for all vCPUs if single-threaded

[Qemu-devel] [PATCH] cpus: run work items for all vCPUs if single-threaded

[Paolo Bonzini]

This avoids the following deadlock:

1) a thread calls run_on_cpu for CPU 2 from a timer, and single_tcg_halt_cond
is signaled

2) CPU 1 is running and exits.  It finds no work item and enters CPU 2

3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
timer never triggers, and CPU 2 never runs the work item

4) run_on_cpu never completes

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 cpus.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/cpus.c b/cpus.c
index a2b33ccb29..0ddeeefc14 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1220,16 +1220,20 @@ static void qemu_wait_io_event_common(CPUState *cpu)
     process_queued_cpu_work(cpu);
 }
 
-static void qemu_tcg_rr_wait_io_event(CPUState *cpu)
+static void qemu_tcg_rr_wait_io_event(void)
 {
+    CPUState *cpu;
+
     while (all_cpu_threads_idle()) {
         stop_tcg_kick_timer();
-        qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
+        qemu_cond_wait(first_cpu->halt_cond, &qemu_global_mutex);
     }
 
     start_tcg_kick_timer();
 
-    qemu_wait_io_event_common(cpu);
+    CPU_FOREACH(cpu) {
+        qemu_wait_io_event_common(cpu);
+    }
 }
 
 static void qemu_wait_io_event(CPUState *cpu)
@@ -1562,7 +1566,7 @@ static void *qemu_tcg_rr_cpu_thread_fn(void *arg)
             qemu_notify_event();
         }
 
-        qemu_tcg_rr_wait_io_event(cpu ? cpu : first_cpu);
+        qemu_tcg_rr_wait_io_event();
         deal_with_unplugged_cpus();
     }
 
-- 
2.17.1

Re: [Qemu-devel] [PATCH] cpus: run work items for all vCPUs if single-threaded

[Emilio G. Cota]

On Wed, Nov 14, 2018 at 12:44:00 +0100, Paolo Bonzini wrote:
> This avoids the following deadlock:
> 
> 1) a thread calls run_on_cpu for CPU 2 from a timer, and single_tcg_halt_cond
> is signaled
> 
> 2) CPU 1 is running and exits.  It finds no work item and enters CPU 2
> 
> 3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
> timer never triggers, and CPU 2 never runs the work item
> 
> 4) run_on_cpu never completes

I'm having trouble understanding (2)->(3).

When the vCPU thread enters CPU 2, shouldn't it detect that work is
pending? As in:

	/* assume cpu == cpu2 in the example above */
	while (cpu && !cpu->queued_work_first && !cpu->exit_request) {

Both cpu->queued_work_first and cpu->exit_request will be set for cpu2.

I can see though how with an additional CPU the deadlock
could happen. For example, the I/O thread does run_on_cpu(cpu3),
which kicks cpu1 (i.e. the tcg_current_rr_cpu) and cpu3, but not cpu2.
Then cpu1 exits, and cpu2 starts executing; unless cpu2 exits on its
own volition, it will run forever.

Thanks,

		Emilio

Re: [Qemu-devel] [PATCH] cpus: run work items for all vCPUs if single-threaded

[Paolo Bonzini]

On 14/11/2018 20:42, Emilio G. Cota wrote:
> On Wed, Nov 14, 2018 at 12:44:00 +0100, Paolo Bonzini wrote:
>> This avoids the following deadlock:
>>
>> 1) a thread calls run_on_cpu for CPU 2 from a timer, and single_tcg_halt_cond
>> is signaled
>>
>> 2) CPU 1 is running and exits.  It finds no work item and enters CPU 2
>>
>> 3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
>> timer never triggers, and CPU 2 never runs the work item
>>
>> 4) run_on_cpu never completes
> 
> I'm having trouble understanding (2)->(3).
> 
> When the vCPU thread enters CPU 2, shouldn't it detect that work is
> pending? As in:
> 
> 	/* assume cpu == cpu2 in the example above */
> 	while (cpu && !cpu->queued_work_first && !cpu->exit_request) {
> 
> Both cpu->queued_work_first and cpu->exit_request will be set for cpu2.
> 
> I can see though how with an additional CPU the deadlock
> could happen. For example, the I/O thread does run_on_cpu(cpu3),
> which kicks cpu1 (i.e. the tcg_current_rr_cpu) and cpu3, but not cpu2.
> Then cpu1 exits, and cpu2 starts executing; unless cpu2 exits on its
> own volition, it will run forever.

Yes, the thread must call run_on_cpu for CPU *3* from a timer.

Paolo

Re: [Qemu-devel] [PATCH] cpus: run work items for all vCPUs if single-threaded

[Emilio G. Cota]

On Fri, Nov 16, 2018 at 00:15:53 +0100, Paolo Bonzini wrote:
> On 14/11/2018 20:42, Emilio G. Cota wrote:
> > On Wed, Nov 14, 2018 at 12:44:00 +0100, Paolo Bonzini wrote:
> >> This avoids the following deadlock:
> >>
> >> 1) a thread calls run_on_cpu for CPU 2 from a timer, and single_tcg_halt_cond
> >> is signaled
> >>
> >> 2) CPU 1 is running and exits.  It finds no work item and enters CPU 2
> >>
> >> 3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
> >> timer never triggers, and CPU 2 never runs the work item
> >>
> >> 4) run_on_cpu never completes
(snip)
> > I can see though how with an additional CPU the deadlock
> > could happen. For example, the I/O thread does run_on_cpu(cpu3),
> > which kicks cpu1 (i.e. the tcg_current_rr_cpu) and cpu3, but not cpu2.
> > Then cpu1 exits, and cpu2 starts executing; unless cpu2 exits on its
> > own volition, it will run forever.
> 
> Yes, the thread must call run_on_cpu for CPU *3* from a timer.

Thanks! Please add my

Reviewed-by: Emilio G. Cota <cota@braap.org>

tag when fixing up the commit message.

		E.