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Re: [Qemu-devel] [PATCH 02/12] trace+libvirt: start trace processing thr
From: |
Michael Mueller |
Subject: |
Re: [Qemu-devel] [PATCH 02/12] trace+libvirt: start trace processing thread in final child process |
Date: |
Fri, 6 Sep 2013 16:06:08 +0200 |
On Wed, 17 Jul 2013 10:08:15 +0800
Stefan Hajnoczi <address@hidden> wrote:
> On Tue, Jul 16, 2013 at 02:17:28PM +0200, Michael Mueller wrote:
> > On Tue, 16 Jul 2013 11:05:11 +0800
> > Stefan Hajnoczi <address@hidden> wrote:
> >
> > > On Mon, Jul 15, 2013 at 09:41:19PM +0200, Christian Borntraeger wrote:
> > > > When running with trace backend e.g. "simple" the writer thread
> > > > needs to be implemented in the same process context as the trace
> > > > points that will be processed. Under libvirtd control, qemu gets
> > > > first started in daemonized mode to privide its capabilities.
> > > > Creating the writer thread in the initial process context then
> > > > leads to a dead lock because the thread gets termined together with
> > > > the initial parent. (-daemonize) This results in stale qemu
> > > > processes. Fix this by deferring trace initialization.
> > >
> > > I don't think this works since trace events will fill up trace_buf[]
> > > and eventually invoke flush_trace_file().
> > >
> > > At that point we use trace_available_cond and trace_empty_cond, which
> > > may be NULL in Glib <2.31.0.
> > >
> > > Perhaps this can be made safe by checking trace_writeout_enabled. It
> > > will be false before the backend has been initialized.
> > >
> > > Stefan
> > >
> >
> > You mean something like this. I'll give it a try:
> >
> > ---
> > trace/simple.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > --- a/trace/simple.c
> > +++ b/trace/simple.c
> > @@ -53,7 +53,7 @@ static GCond *trace_empty_cond;
> > #endif
> >
> > static bool trace_available;
> > -static bool trace_writeout_enabled;
> > +static bool trace_writeout_enabled = false;
>
> static bool is automatically initialized to false.
>
> > enum {
> > TRACE_BUF_LEN = 4096 * 64,
> > @@ -427,5 +427,6 @@ bool trace_backend_init(const char *even
> > atexit(st_flush_trace_buffer);
> > trace_backend_init_events(events);
> > st_set_trace_file(file);
> > + trace_writeout_enabled = false;
>
> I was thinking along the lines of trace_record_finish() not calling
> flush_trace_file() if trace_writeout_enabled is false.
>
> Stefan
>
I just looked into it again and think that it is save the way I suggested,
because as long
trace_backend_init() isn't called, also trace_backend_init_events() hasn't
registered any
events. Thus no trace records will be written and can fill up the trace buffer.
Michael
- Re: [Qemu-devel] [PATCH 02/12] trace+libvirt: start trace processing thread in final child process,
Michael Mueller <=