Re: [PATCH v2 06/14] block: remove AioContext locking

[Kevin Wolf <kwolf@xxxxxxxxxx>]

Am 05.12.2023 um 19:20 hat Stefan Hajnoczi geschrieben:
> This is the big patch that removes
> aio_context_acquire()/aio_context_release() from the block layer and
> affected block layer users.
> 
> There isn't a clean way to split this patch and the reviewers are likely
> the same group of people, so I decided to do it in one patch.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>
> Reviewed-by: Kevin Wolf <kwolf@xxxxxxxxxx>
> Reviewed-by: Paul Durrant <paul@xxxxxxx>

> diff --git a/migration/block.c b/migration/block.c
> index a15f9bddcb..2bcfcbfdf6 100644
> --- a/migration/block.c
> +++ b/migration/block.c
> @@ -313,22 +311,10 @@ static int mig_save_device_bulk(QEMUFile *f, 
> BlkMigDevState *bmds)
>      block_mig_state.submitted++;
>      blk_mig_unlock();
>  
> -    /* We do not know if bs is under the main thread (and thus does
> -     * not acquire the AioContext when doing AIO) or rather under
> -     * dataplane.  Thus acquire both the iothread mutex and the
> -     * AioContext.
> -     *
> -     * This is ugly and will disappear when we make bdrv_* thread-safe,
> -     * without the need to acquire the AioContext.
> -     */
> -    qemu_mutex_lock_iothread();
> -    aio_context_acquire(blk_get_aio_context(bmds->blk));
>      bdrv_reset_dirty_bitmap(bmds->dirty_bitmap, cur_sector * 
> BDRV_SECTOR_SIZE,
>                              nr_sectors * BDRV_SECTOR_SIZE);
>      blk->aiocb = blk_aio_preadv(bb, cur_sector * BDRV_SECTOR_SIZE, 
> &blk->qiov,
>                                  0, blk_mig_read_cb, blk);
> -    aio_context_release(blk_get_aio_context(bmds->blk));
> -    qemu_mutex_unlock_iothread();
>  
>      bmds->cur_sector = cur_sector + nr_sectors;
>      return (bmds->cur_sector >= total_sectors);

With this hunk applied, qemu-iotests 183 fails:

(gdb) bt
#0  0x000055aaa7d47c09 in bdrv_graph_co_rdlock () at ../block/graph-lock.c:176
#1  0x000055aaa7d3de2e in graph_lockable_auto_lock (x=<optimized out>) at 
/home/kwolf/source/qemu/include/block/graph-lock.h:215
#2  blk_co_do_preadv_part (blk=0x7f38a4000f30, offset=0, bytes=1048576, 
qiov=0x7f38a40250f0, qiov_offset=qiov_offset@entry=0, flags=0) at 
../block/block-backend.c:1340
#3  0x000055aaa7d3e006 in blk_aio_read_entry (opaque=0x7f38a4025140) at 
../block/block-backend.c:1620
#4  0x000055aaa7e7aa5b in coroutine_trampoline (i0=<optimized out>, 
i1=<optimized out>) at ../util/coroutine-ucontext.c:177
#5  0x00007f38d14dbe90 in __start_context () at /lib64/libc.so.6
#6  0x00007f38b3dfa060 in  ()
#7  0x0000000000000000 in  ()

qemu_get_current_aio_context() returns NULL now. I don't completely
understand why it depends on the BQL, but adding the BQL locking back
fixes it.

Kevin