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Re: [Qemu-devel] [PULL for-3.0 1/1] trace/simple: fix hang in child afte

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PULL for-3.0 1/1] trace/simple: fix hang in child after fork(2)
Date: Tue, 24 Jul 2018 15:41:01 +0100
User-agent: Mutt/1.10.0 (2018-05-17)

On Tue, Jul 24, 2018 at 03:35:51PM +0100, Daniel P. Berrangé wrote:
> On Tue, Jul 24, 2018 at 03:25:04PM +0100, Stefan Hajnoczi 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.
> I'm not convinced this is safe, as it looks like it has a window in
> which both the parent and child processes will be doing write-out to
> the same file.
> In particular in the main QEMU system emulators it means that any
> time we fork() in QEMU, eg for spawning commands with migration
> exec: URI, or TAP devuce ifup scripts, etc, we'll be starting a
> write-out thread in the child.

I'd be more inclined to have the pthread_atfork() handle simply terminate
the tracing process, reversing all effects of trace_init_backends(). Then
after qemu-nbd has called fork(), it can simply call trace_init_backends()
explicitly to start it running again. This avoids unecessarily starting
tracing in child processes that are not requiring/expecting it.

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