Re: [Qemu-devel] [PATCH v2 4/4] migration: use bdrv_drain_all_begin/end() instead bdrv_drain_all()

[Stefan Hajnoczi]

On Mon, May 22, 2017 at 02:17:35PM +0200, Kevin Wolf wrote:
> Am 19.05.2017 um 12:32 hat Stefan Hajnoczi geschrieben:
> > blk/bdrv_drain_all() only takes effect for a single instant and then
> > resumes block jobs, guest devices, and other external clients like the
> > NBD server.  This can be handy when performing a synchronous drain
> > before terminating the program, for example.
> > 
> > Monitor commands usually need to quiesce I/O across an entire code
> > region so blk/bdrv_drain_all() is not suitable.  They must use
> > bdrv_drain_all_begin/end() to mark the region.  This prevents new I/O
> > requests from slipping in or worse - block jobs completing and modifying
> > the graph.
> > 
> > I audited other blk/bdrv_drain_all() callers but did not find anything
> > that needs a similar fix.  This patch fixes the savevm/loadvm commands.
> > Although I haven't encountered a read world issue this makes the code
> > safer.
> > 
> > Suggested-by: Kevin Wolf <address@hidden>
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> 
> > @@ -2279,7 +2284,7 @@ int load_vmstate(const char *name, Error **errp)
> >      }
> >  
> >      /* Flush all IO requests so they don't interfere with the new state.  
> > */
> > -    bdrv_drain_all();
> > +    bdrv_drain_all_begin();
> >  
> >      ret = bdrv_all_goto_snapshot(name, &bs);
> >      if (ret < 0) {
> > @@ -2303,6 +2308,8 @@ int load_vmstate(const char *name, Error **errp)
> >      qemu_fclose(f);
> >      aio_context_release(aio_context);
> >  
> > +    bdrv_drain_all_end();
> > +
> >      migration_incoming_state_destroy();
> >      if (ret < 0) {
> >          error_setg(errp, "Error %d while loading VM state", ret);
> 
> There are a few error returns between these two places where a
> bdrv_drain_all_end() is missing.

Thank you, will fix in the next revision.

Stefan

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