Re: [PATCH v8 3/7] 9pfs: split out fs driver core of v9fs_co_readdir()

[Christian Schoenebeck]

On Mittwoch, 29. Juli 2020 18:02:56 CEST Greg Kurz wrote:
> On Wed, 29 Jul 2020 10:11:54 +0200
> 
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > The implementation of v9fs_co_readdir() has two parts: the outer
> > part is executed by main I/O thread, whereas the inner part is
> > executed by fs driver on a background I/O thread.
> > 
> > Move the inner part to its own new, private function do_readdir(),
> > so it can be shared by another upcoming new function.
> > 
> > This is just a preparatory patch for the subsequent patch, with the
> > purpose to avoid the next patch to clutter the overall diff.
> > 
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
> > 
> >  hw/9pfs/codir.c | 37 +++++++++++++++++++++++--------------
> >  1 file changed, 23 insertions(+), 14 deletions(-)
> > 
> > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c
> > index 73f9a751e1..ff57fb8619 100644
> > --- a/hw/9pfs/codir.c
> > +++ b/hw/9pfs/codir.c
> > @@ -18,28 +18,37 @@
> > 
> >  #include "qemu/main-loop.h"
> >  #include "coth.h"
> > 
> > +/*
> > + * This must solely be executed on a background IO thread.
> > + */
> 
> Well, technically this function could be called from any context
> but of course calling it from the main I/O thread when handling
> T_readdir would make the request synchronous, which is certainly
> not what we want. So I'm not sure this comment brings much.

Yeah, the intention was to more clearly separate functions' intended usage 
context either being TH or rather BH focused, by sticking appropriate human-
readable API comments to them.

But you are right, the TH functions are more limited in this regards as they 
usually contain a co-routine dispatch call, whereas BH functions usually 
preserve a more flexible/universal nature.

So maybe rather:

/*
 * Intended to be called from bottom-half (e.g. background I/O thread) 
 * context.
 */

On doubt I can also just drop the comment, as the function is really quite 
simple.

> Anyway, the code change is okay so:
> 
> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> > +static int do_readdir(V9fsPDU *pdu, V9fsFidState *fidp, struct dirent
> > **dent) +{
> > +    int err = 0;
> > +    V9fsState *s = pdu->s;
> > +    struct dirent *entry;
> > +
> > +    errno = 0;
> > +    entry = s->ops->readdir(&s->ctx, &fidp->fs);
> > +    if (!entry && errno) {
> > +        *dent = NULL;
> > +        err = -errno;
> > +    } else {
> > +        *dent = entry;
> > +    }
> > +    return err;
> > +}
> > +
> > 
> >  int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
> >  
> >                                   struct dirent **dent)
> >  
> >  {
> >  
> >      int err;
> > 
> > -    V9fsState *s = pdu->s;
> > 
> >      if (v9fs_request_cancelled(pdu)) {
> >      
> >          return -EINTR;
> >      
> >      }
> > 
> > -    v9fs_co_run_in_worker(
> > -        {
> > -            struct dirent *entry;
> > -
> > -            errno = 0;
> > -            entry = s->ops->readdir(&s->ctx, &fidp->fs);
> > -            if (!entry && errno) {
> > -                err = -errno;
> > -            } else {
> > -                *dent = entry;
> > -                err = 0;
> > -            }
> > -        });
> > +    v9fs_co_run_in_worker({
> > +        err = do_readdir(pdu, fidp, dent);
> > +    });
> > 
> >      return err;
> >  
> >  }