Re: [Qemu-devel] RFC [PATCH] Make bdrv_flush synchronous only and update callers

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

  I am glad to have an accurate sync bdrv_flush(). Code looks fine.

Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>

> This patch makes bdrv_flush a synchronous function and updates any callers from
> a coroutine context to use bdrv_co_flush instead.
> 
> The motivation for this patch comes from the GSoC Continuation-Passing C
> project. When coroutines were introduced, synchronous functions in the block
> layer were converted to use asynchronous methods by dynamically detecting if
> they were being run from a coroutine context by calling qemu_in_coroutine(), and
> yielding if so. If they were not, they would spawn a new coroutine and poll
> until the asynchronous counterpart finished.
> 
> However this approach does not work with CPC as the CPC translator converts all
> functions annotated coroutine_fn to a different (continuation based) calling
> convention. This means that coroutine_fn annotated functions cannot be called
> from a non-coroutine context.
> 
> This patch is a Request For Comments on the approach of splitting these
> "dynamic" functions into synchronous and asynchronous versions. This is easy for
> bdrv_flush as it already has an asynchronous counterpart - bdrv_co_flush. The
> only caller of bdrv_flush from a coroutine context is mirror_drain in
> block/mirror.c - this should be annotated as a coroutine_fn as it calls
> qemu_coroutine_yield().
> 
> If this approach meets with approval I will develop a patchset splitting the
> other "dynamic" functions in the block layer. This will allow all coroutine
> functions to have a coroutine_fn annotation that can be statically checked (CPC
> can be used to verify annotations).
> 
> I have audited the other callers of bdrv_flush, they are included below:
> 
> block.c: bdrv_reopen_prepare, bdrv_close, bdrv_commit, bdrv_pwrite_sync
> block/qcow2-cache.c: qcow2_cache_entry_flush, qcow2_cache_flush
> block/qcow2-refcount.c: qcow2_update_snapshot_refcount
> block/qcow2-snapshot.c: qcow2_write_snapshots
> block/qcow2.c: qcow2_mark_dirty, qcow2_mark_clean
> block/qed-check.c: qed_check_mark_clean
> block/qed.c: bdrv_qed_open, bdrv_qed_close
> blockdev.c: external_snapshot_prepare, do_drive_del
> cpus.c: do_vm_stop
> hw/block/nvme.c: nvme_clear_ctrl
> qemu-io-cmds.c: flush_f
> savevm.c: bdrv_fclose
> 
> ---
>   block.c        | 13 ++++---------
>   block/mirror.c |  4 ++--
>   2 files changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 6c493ad..00d71df 100644
> --- a/block.c
> +++ b/block.c
> @@ -4110,15 +4110,10 @@ int bdrv_flush(BlockDriverState *bs)
>           .ret = NOT_DONE,
>       };
> 
> -    if (qemu_in_coroutine()) {
> -        /* Fast-path if already in coroutine context */
> -        bdrv_flush_co_entry(&rwco);
> -    } else {
> -        co = qemu_coroutine_create(bdrv_flush_co_entry);
> -        qemu_coroutine_enter(co, &rwco);
> -        while (rwco.ret == NOT_DONE) {
> -            qemu_aio_wait();
> -        }
> +    co = qemu_coroutine_create(bdrv_flush_co_entry);
> +    qemu_coroutine_enter(co, &rwco);
> +    while (rwco.ret == NOT_DONE) {
> +        qemu_aio_wait();
>       }
> 
>       return rwco.ret;
> diff --git a/block/mirror.c b/block/mirror.c
> index bed4a7e..3d5da7e 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -282,7 +282,7 @@ static void mirror_free_init(MirrorBlockJob *s)
>       }
>   }
> 
> -static void mirror_drain(MirrorBlockJob *s)
> +static void coroutine_fn mirror_drain(MirrorBlockJob *s)
>   {
>       while (s->in_flight > 0) {
>           qemu_coroutine_yield();
> @@ -390,7 +390,7 @@ static void coroutine_fn mirror_run(void *opaque)
>           should_complete = false;
>           if (s->in_flight == 0 && cnt == 0) {
>               trace_mirror_before_flush(s);
> -            ret = bdrv_flush(s->target);
> +            ret = bdrv_co_flush(s->target);
>               if (ret < 0) {
>                   if (mirror_error_action(s, false, -ret) == BDRV_ACTION_REPORT) {
>                       goto immediate_exit;
> 


-- 
Best Regards

Wenchao Xia