On Fri, 09/07 18:15, Kevin Wolf wrote:
> bdrv_do_drained_begin/end() assume that they are called with the
> AioContext lock of bs held. If we call drain functions from a coroutine
> with the AioContext lock held, we yield and schedule a BH to move out of
> coroutine context. This means that the lock for the home context of the
> coroutine is released and must be re-acquired in the bottom half.
>
> Signed-off-by: Kevin Wolf <[email protected]>
> ---
> include/qemu/coroutine.h | 5 +++++
> block/io.c | 15 +++++++++++++++
> util/qemu-coroutine.c | 5 +++++
> 3 files changed, 25 insertions(+)
>
> diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
> index 6f8a487041..9801e7f5a4 100644
> --- a/include/qemu/coroutine.h
> +++ b/include/qemu/coroutine.h
> @@ -90,6 +90,11 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine
> *co);
> void coroutine_fn qemu_coroutine_yield(void);
>
> /**
> + * Get the AioContext of the given coroutine
> + */
> +AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
> +
> +/**
> * Get the currently executing coroutine
> */
> Coroutine *coroutine_fn qemu_coroutine_self(void);
> diff --git a/block/io.c b/block/io.c
> index 7100344c7b..914ba78f1a 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -288,6 +288,18 @@ static void bdrv_co_drain_bh_cb(void *opaque)
> BlockDriverState *bs = data->bs;
>
> if (bs) {
> + AioContext *ctx = bdrv_get_aio_context(bs);
> + AioContext *co_ctx = qemu_coroutine_get_aio_context(co);
> +
> + /*
> + * When the coroutine yielded, the lock for its home context was
> + * released, so we need to re-acquire it here. If it explicitly
> + * acquired a different context, the lock is still held and we don't
> + * want to lock it a second time (or AIO_WAIT_WHILE() would hang).
> + */
This condition is rather obscure. When is ctx not equal to co_ctx?
> + if (ctx == co_ctx) {
> + aio_context_acquire(ctx);
> + }
> bdrv_dec_in_flight(bs);
> if (data->begin) {
> bdrv_do_drained_begin(bs, data->recursive, data->parent,
> @@ -296,6 +308,9 @@ static void bdrv_co_drain_bh_cb(void *opaque)
> bdrv_do_drained_end(bs, data->recursive, data->parent,
> data->ignore_bds_parents);
> }
> + if (ctx == co_ctx) {
> + aio_context_release(ctx);
> + }
> } else {
> assert(data->begin);
> bdrv_drain_all_begin();
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 1ba4191b84..2295928d33 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -198,3 +198,8 @@ bool qemu_coroutine_entered(Coroutine *co)
> {
> return co->caller;
> }
> +
> +AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co)
> +{
> + return co->ctx;
> +}
> --
> 2.13.6
>
