[Qemu-devel] Re: [v1 PATCH 3/3]: Convert v9fs_stat to threaded model.

[Stefan Hajnoczi]

On Tue, Mar 15, 2011 at 10:39 AM, Arun R Bharadwaj
<address@hidden> wrote:
> -static void v9fs_stat_post_lstat(V9fsState *s, V9fsStatState *vs, int err)
> +static void v9fs_stat_post_lstat(void *opaque)
>  {
> -    if (err == -1) {
> -        err = -errno;
> +    V9fsStatState *vs = (V9fsStatState *)opaque;

No need to cast void* in C.

> +    if (vs->err == -1) {
> +        vs->err = -(vs->v9fs_errno);

How about the thread worker function puts the -errno into a vs->ret field:

static void v9fs_stat_do_lstat(V9fsRequest *request)
{
    V9fsStatState *vs = container_of(request, V9fsStatState, request);

    vs->ret = v9fs_do_lstat(vs->s, &vs->fidp->fsmap.path, &vs->stbuf);
    if (vs->ret != 0) {
        vs->ret = -errno;
    }
}

Then v9fs_stat_post_lstat() can use vs->ret directly and does not need
to juggle around the two fields, vs->err and vs->v9fs_errno.

>         goto out;
>     }
>
> -    err = stat_to_v9stat(s, &vs->fidp->fsmap.path, &vs->stbuf, &vs->v9stat);
> -    if (err) {
> +    vs->err = stat_to_v9stat(vs->s, &vs->fidp->fsmap.path, &vs->stbuf, 
> &vs->v9stat);

This function can block in v9fs_do_readlink().  Needs to be done
asynchronously to avoid blocking QEMU.

> +    if (vs->err) {
>         goto out;
>     }
>     vs->offset += pdu_marshal(vs->pdu, vs->offset, "wS", 0, &vs->v9stat);
> -    err = vs->offset;
> +    vs->err = vs->offset;
>
>  out:
> -    complete_pdu(s, vs->pdu, err);
> +    complete_pdu(vs->s, vs->pdu, vs->err);
>     v9fs_stat_free(&vs->v9stat);
>     qemu_free(vs);
>  }
>
> +static void v9fs_stat_do_lstat(V9fsRequest *request)
> +{
> +    V9fsStatState *vs = container_of(request, V9fsStatState, request);

Nice.  Could container_of() be used for v9fs_post_lstat() too?  I'm
suggesting making post op functions take the V9fsRequest* instead of a
void* opaque pointer.

> +
> +    vs->err = v9fs_do_lstat(vs->s, &vs->fidp->fsmap.path, &vs->stbuf);

This is not threadsafe since rpath still uses a static buffer in
qemu.git.  Please ensure that rpath() is thread-safe before pushing
this patch.

> +    vs->v9fs_errno = errno;
> +}
> +
>  static void v9fs_stat(V9fsState *s, V9fsPDU *pdu)
>  {
>     int32_t fid;
> @@ -1487,6 +1496,10 @@ static void v9fs_stat(V9fsState *s, V9fsPDU *pdu)
>     vs = qemu_malloc(sizeof(*vs));
>     vs->pdu = pdu;
>     vs->offset = 7;
> +    vs->s = s;
> +    vs->request.func = v9fs_stat_do_lstat;
> +    vs->request.post_op.func = v9fs_stat_post_lstat;
> +    vs->request.post_op.arg = vs;
>
>     memset(&vs->v9stat, 0, sizeof(vs->v9stat));
>
> @@ -1498,8 +1511,11 @@ static void v9fs_stat(V9fsState *s, V9fsPDU *pdu)
>         goto out;
>     }
>
> +    /*
>     err = v9fs_do_lstat(s, &vs->fidp->fsmap.path, &vs->stbuf);
>     v9fs_stat_post_lstat(s, vs, err);
> +    */

Please remove unused code, it quickly becomes out-of-date and confuses readers.

> +    v9fs_qemu_submit_request(&vs->request);

What happens when another PDU is handled next that uses the same fid?
The worst case is if the client sends TCLUNK and fid is freed while
the worker thread and later the post op still access the memory.
There needs to be some kind of guard (like a reference count) to
prevent this.

Stefan