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;
--
1.8.3.2
