Re: [PATCH v4 1/6] blkdebug: refactor removal of a suspended request

[Eric Blake <eblake@redhat.com>]

On Fri, Jun 04, 2021 at 12:07:36PM +0200, Emanuele Giuseppe Esposito wrote:
> Extract to a separate function.  Do not rely on FOREACH_SAFE, which is
> only "safe" if the *current* node is removed---not if another node is
> removed.  Instead, just walk the entire list from the beginning when
> asked to resume all suspended requests with a given tag.
> 
> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/blkdebug.c | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blkdebug.c b/block/blkdebug.c
> index 2c0b9b0ee8..8f19d991fa 100644
> --- a/block/blkdebug.c

> -static int blkdebug_debug_resume(BlockDriverState *bs, const char *tag)
> +static int resume_req_by_tag(BDRVBlkdebugState *s, const char *tag, bool all)
>  {
> -    BDRVBlkdebugState *s = bs->opaque;
> -    BlkdebugSuspendedReq *r, *next;
> +    BlkdebugSuspendedReq *r;
>  
> -    QLIST_FOREACH_SAFE(r, &s->suspended_reqs, next, next) {
> +retry:
> +    QLIST_FOREACH(r, &s->suspended_reqs, next) {
>          if (!strcmp(r->tag, tag)) {
> +            QLIST_REMOVE(r, next);

Isn't the whole point of using QLIST_FOREACH_SAFE the ability to call
QLIST_REMOVE on an element in that list while still iterating?

>              qemu_coroutine_enter(r->co);
> +            if (all) {
> +                goto retry;
> +            }
>              return 0;

Oh, I see - you abandon the iteration in all control flow paths, so
the simpler loop is still okay.  Still, this confused me enough on
first read that it may be worth a comment, maybe:

/* No need for _SAFE, because iteration stops on first hit */

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org