Re: [Qemu-devel] [PATCH v3] trace/simple: fix hang in child after fork(2)

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 17 Jul 2018 11:19:44 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> The simple trace backend spawns a write-out thread which is used to
> asynchronously flush the in-memory ring buffer to disk.
> 
> fork(2) does not clone all threads, only the thread that invoked
> fork(2).  As a result there is no write-out thread in the child process!
> 
> This causes a hang during shutdown when atexit(3) handler installed by
> the simple trace backend waits for the non-existent write-out thread.
> 
> This patch uses pthread_atfork(3) to terminate the write-out thread
> before fork and restart it in both the parent and child after fork.
> This solves a hang in qemu-iotests 147 due to qemu-nbd --fork usage.
> 
> Reported-by: Cornelia Huck <cohuck@redhat.com>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> v3:
>  * Hold trace_lock across fork() to prevent possibility of another
>    thread holding it and disappearing [Paolo]
> 
>  trace/simple.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 71 insertions(+), 9 deletions(-)

Tested-by: Cornelia Huck <cohuck@redhat.com>