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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] RFC [PATCH] Make bdrv_flush synchronous only and update callers
Date: Fri, 19 Jul 2013 13:27:44 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Jul 18, 2013 at 11:21:42PM +0200, Charlie Shepherd wrote:
> 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

bdrv_pwrite_sync() is a dynamic function.  If called from coroutine
context it will yield (indirectly from bdrv_pwrite() or bdrv_flush()).

> 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

qcow2 runs in coroutine context, the coroutine_fn annotations are just
missing.  See block/qcow2.c:bdrv_qcow2 for the entry points like

> 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

I think the rest are fine and your patch looks good.

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