[PATCH] runstate: ignore finishmigrate -> prelaunch transition

Laurent Vivier posted 1 patch 2 weeks ago
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git fetch https://github.com/patchew-project/qemu tags/patchew/20191129115132.285988-1-lvivier@redhat.com
Maintainers: Paolo Bonzini <pbonzini@redhat.com>
vl.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

[PATCH] runstate: ignore finishmigrate -> prelaunch transition

Posted by Laurent Vivier 2 weeks ago
Commit 1bd71dce4bf2 tries to prevent a finishmigrate -> prelaunch
transition by exiting at the beginning of the main_loop_should_exit()
function if the state is already finishmigrate.

As the finishmigrate state is set in the migration thread it can
happen concurrently to the function. The migration thread and the
function are normally protected by the iothread mutex and thus the
state should no evolve between the start of the function and its end.

Unfortunately during the function life the lock is released by
pause_all_vcpus() just before the point we need to be sure we are
not in finishmigrate state and if the migration thread is waiting
for the lock it will take the opportunity to change the state
to finishmigrate.

The only way to be sure we are not in the finishmigrate state when
we need is to check the state after the pause_all_vcpus() function.

Fixes: 1bd71dce4bf2 ("runstate: ignore exit request in finish migrate state")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 vl.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/vl.c b/vl.c
index 6a65a64bfd64..bf0a6345d239 100644
--- a/vl.c
+++ b/vl.c
@@ -1745,9 +1745,6 @@ static bool main_loop_should_exit(void)
     RunState r;
     ShutdownCause request;
 
-    if (runstate_check(RUN_STATE_FINISH_MIGRATE)) {
-        return false;
-    }
     if (preconfig_exit_requested) {
         if (runstate_check(RUN_STATE_PRECONFIG)) {
             runstate_set(RUN_STATE_PRELAUNCH);
@@ -1776,8 +1773,13 @@ static bool main_loop_should_exit(void)
         pause_all_vcpus();
         qemu_system_reset(request);
         resume_all_vcpus();
+        /*
+         * runstate can change in pause_all_vcpus()
+         * as iothread mutex is unlocked
+         */
         if (!runstate_check(RUN_STATE_RUNNING) &&
-                !runstate_check(RUN_STATE_INMIGRATE)) {
+                !runstate_check(RUN_STATE_INMIGRATE) &&
+                !runstate_check(RUN_STATE_FINISH_MIGRATE)) {
             runstate_set(RUN_STATE_PRELAUNCH);
         }
     }
-- 
2.23.0


Re: [PATCH] runstate: ignore finishmigrate -> prelaunch transition

Posted by Dr. David Alan Gilbert 1 week ago
* Laurent Vivier (lvivier@redhat.com) wrote:
> Commit 1bd71dce4bf2 tries to prevent a finishmigrate -> prelaunch
> transition by exiting at the beginning of the main_loop_should_exit()
> function if the state is already finishmigrate.
> 
> As the finishmigrate state is set in the migration thread it can
> happen concurrently to the function. The migration thread and the
> function are normally protected by the iothread mutex and thus the
> state should no evolve between the start of the function and its end.
> 
> Unfortunately during the function life the lock is released by
> pause_all_vcpus() just before the point we need to be sure we are
> not in finishmigrate state and if the migration thread is waiting
> for the lock it will take the opportunity to change the state
> to finishmigrate.

Ewww.
I hate those short wakeups for pause_all_vcpus; I'm sure there are loads
more corners that break.

Still, I _think_ this is an improvement, so:

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> The only way to be sure we are not in the finishmigrate state when
> we need is to check the state after the pause_all_vcpus() function.
> 
> Fixes: 1bd71dce4bf2 ("runstate: ignore exit request in finish migrate state")
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  vl.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/vl.c b/vl.c
> index 6a65a64bfd64..bf0a6345d239 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1745,9 +1745,6 @@ static bool main_loop_should_exit(void)
>      RunState r;
>      ShutdownCause request;
>  
> -    if (runstate_check(RUN_STATE_FINISH_MIGRATE)) {
> -        return false;
> -    }
>      if (preconfig_exit_requested) {
>          if (runstate_check(RUN_STATE_PRECONFIG)) {
>              runstate_set(RUN_STATE_PRELAUNCH);
> @@ -1776,8 +1773,13 @@ static bool main_loop_should_exit(void)
>          pause_all_vcpus();
>          qemu_system_reset(request);
>          resume_all_vcpus();
> +        /*
> +         * runstate can change in pause_all_vcpus()
> +         * as iothread mutex is unlocked
> +         */
>          if (!runstate_check(RUN_STATE_RUNNING) &&
> -                !runstate_check(RUN_STATE_INMIGRATE)) {
> +                !runstate_check(RUN_STATE_INMIGRATE) &&
> +                !runstate_check(RUN_STATE_FINISH_MIGRATE)) {
>              runstate_set(RUN_STATE_PRELAUNCH);
>          }
>      }
> -- 
> 2.23.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK


Re: [PATCH] runstate: ignore finishmigrate -> prelaunch transition

Posted by Laurent Vivier 2 days ago
On 06/12/2019 20:52, Dr. David Alan Gilbert wrote:
> * Laurent Vivier (lvivier@redhat.com) wrote:
>> Commit 1bd71dce4bf2 tries to prevent a finishmigrate -> prelaunch
>> transition by exiting at the beginning of the main_loop_should_exit()
>> function if the state is already finishmigrate.
>>
>> As the finishmigrate state is set in the migration thread it can
>> happen concurrently to the function. The migration thread and the
>> function are normally protected by the iothread mutex and thus the
>> state should no evolve between the start of the function and its end.
>>
>> Unfortunately during the function life the lock is released by
>> pause_all_vcpus() just before the point we need to be sure we are
>> not in finishmigrate state and if the migration thread is waiting
>> for the lock it will take the opportunity to change the state
>> to finishmigrate.
> 
> Ewww.
> I hate those short wakeups for pause_all_vcpus; I'm sure there are loads
> more corners that break.
> 
> Still, I _think_ this is an improvement, so:
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 

Who volunteers to take this in his queue?

Thanks,
Laurent