Re: [PATCH v2] 9pfs: Improve unreclaim loop

[Greg Kurz]

On Fri, 22 Jan 2021 14:09:12 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Donnerstag, 21. Januar 2021 19:15:10 CET Greg Kurz wrote:
> > If a fid was actually re-opened by v9fs_reopen_fid(), we re-traverse the
> > fid list from the head in case some other request created a fid that
> > needs to be marked unreclaimable as well (ie. the client opened a new
> 
> That's "i.e.". Not a big deal so ...
> 

I'll fix this up before pushing.

> Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> 

Thanks ! I'll post the patch for the reclaim_list conversion shortly.

> > handle on the path that is being unlinked). This is suboptimal since
> > most if not all fids that require it have likely been taken care of
> > already.
> > 
> > This is mostly the result of new fids being added to the head of the
> > list. Since the list is now a QSIMPLEQ, add new fids at the end instead
> > to avoid the need to rewind. Take a reference on the fid to ensure it
> > doesn't go away during v9fs_reopen_fid() and that it can be safely
> > passed to QSIMPLEQ_NEXT() afterwards. Since the associated put_fid()
> > can also yield, same is done with the next fid. So the logic here is
> > to get a reference on a fid and only put it back during the next
> > iteration after we could get a reference on the next fid.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> > v2: - fix typos in changelog
> >     - drop bogus assert()
> > ---
> >  hw/9pfs/9p.c | 46 ++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 32 insertions(+), 14 deletions(-)
> > 
> > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > index b65f320e6518..3864d014b43c 100644
> > --- a/hw/9pfs/9p.c
> > +++ b/hw/9pfs/9p.c
> > @@ -311,7 +311,7 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t
> > fid) * reclaim won't close the file descriptor
> >       */
> >      f->flags |= FID_REFERENCED;
> > -    QSIMPLEQ_INSERT_HEAD(&s->fid_list, f, next);
> > +    QSIMPLEQ_INSERT_TAIL(&s->fid_list, f, next);
> > 
> >      v9fs_readdir_init(s->proto_version, &f->fs.dir);
> >      v9fs_readdir_init(s->proto_version, &f->fs_reclaim.dir);
> > @@ -497,32 +497,50 @@ static int coroutine_fn
> > v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path) {
> >      int err;
> >      V9fsState *s = pdu->s;
> > -    V9fsFidState *fidp;
> > +    V9fsFidState *fidp, *fidp_next;
> > 
> > -again:
> > -    QSIMPLEQ_FOREACH(fidp, &s->fid_list, next) {
> > -        if (fidp->path.size != path->size) {
> > -            continue;
> > -        }
> > -        if (!memcmp(fidp->path.data, path->data, path->size)) {
> > +    fidp = QSIMPLEQ_FIRST(&s->fid_list);
> > +    if (!fidp) {
> > +        return 0;
> > +    }
> > +
> > +    /*
> > +     * v9fs_reopen_fid() can yield : a reference on the fid must be held
> > +     * to ensure its pointer remains valid and we can safely pass it to
> > +     * QSIMPLEQ_NEXT(). The corresponding put_fid() can also yield so
> > +     * we must keep a reference on the next fid as well. So the logic here
> > +     * is to get a reference on a fid and only put it back during the next
> > +     * iteration after we could get a reference on the next fid. Start with
> > +     * the first one.
> > +     */
> > +    for (fidp->ref++; fidp; fidp = fidp_next) {
> > +        if (fidp->path.size == path->size &&
> > +            !memcmp(fidp->path.data, path->data, path->size)) {
> >              /* Mark the fid non reclaimable. */
> >              fidp->flags |= FID_NON_RECLAIMABLE;
> > 
> >              /* reopen the file/dir if already closed */
> >              err = v9fs_reopen_fid(pdu, fidp);
> >              if (err < 0) {
> > +                put_fid(pdu, fidp);
> >                  return err;
> >              }
> > +        }
> > +
> > +        fidp_next = QSIMPLEQ_NEXT(fidp, next);
> > +
> > +        if (fidp_next) {
> >              /*
> > -             * Go back to head of fid list because
> > -             * the list could have got updated when
> > -             * switched to the worker thread
> > +             * Ensure the next fid survives a potential clunk request
> > during +             * put_fid() below and v9fs_reopen_fid() in the next
> > iteration. */
> > -            if (err == 0) {
> > -                goto again;
> > -            }
> > +            fidp_next->ref++;
> >          }
> > +
> > +        /* We're done with this fid */
> > +        put_fid(pdu, fidp);
> >      }
> > +
> >      return 0;
> >  }
> 
> 
>