Re: [PATCH 11/15] block-backend: make queued_requests thread-safe

[Kevin Wolf <kwolf@redhat.com>]

Am 12.12.2022 um 13:59 hat Paolo Bonzini geschrieben:
> Protect quiesce_counter and queued_requests with a QemuMutex, so that
> they are protected from concurrent access in the main thread (for example
> blk_root_drained_end() reached from bdrv_drain_all()) and in the iothread
> (where any I/O operation will call blk_inc_in_flight()).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/block-backend.c | 44 +++++++++++++++++++++++++++++++++++--------
>  1 file changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 627d491d4155..fdf82f1f1599 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -23,6 +23,7 @@
>  #include "qapi/error.h"
>  #include "qapi/qapi-events-block.h"
>  #include "qemu/id.h"
> +#include "qemu/thread.h"
>  #include "qemu/main-loop.h"
>  #include "qemu/option.h"
>  #include "trace.h"
> @@ -85,6 +86,8 @@ struct BlockBackend {
>      NotifierList remove_bs_notifiers, insert_bs_notifiers;
>      QLIST_HEAD(, BlockBackendAioNotifier) aio_notifiers;
>  
> +    /* Protected by quiesce_lock.  */
> +    QemuMutex quiesce_lock;
>      int quiesce_counter;
>      CoQueue queued_requests;
>  
> @@ -372,6 +375,7 @@ BlockBackend *blk_new(AioContext *ctx, uint64_t perm, uint64_t shared_perm)
>  
>      block_acct_init(&blk->stats);
>  
> +    qemu_mutex_init(&blk->quiesce_lock);
>      qemu_co_queue_init(&blk->queued_requests);
>      notifier_list_init(&blk->remove_bs_notifiers);
>      notifier_list_init(&blk->insert_bs_notifiers);
> @@ -490,6 +494,7 @@ static void blk_delete(BlockBackend *blk)
>      assert(QLIST_EMPTY(&blk->insert_bs_notifiers.notifiers));
>      assert(QLIST_EMPTY(&blk->aio_notifiers));
>      QTAILQ_REMOVE(&block_backends, blk, link);
> +    qemu_mutex_destroy(&blk->quiesce_lock);
>      drive_info_del(blk->legacy_dinfo);
>      block_acct_cleanup(&blk->stats);
>      g_free(blk);
> @@ -1451,11 +1456,25 @@ void blk_inc_in_flight(BlockBackend *blk)
>  {
>      IO_CODE();
>      qatomic_inc(&blk->in_flight);
> -    if (!blk->disable_request_queuing) {
> -        /* TODO: this is not thread-safe! */
> +
> +    /*
> +     * Avoid a continuous stream of requests from AIO callbacks, which
> +     * call a user-provided callback while blk->in_flight is elevated,
> +     * if the BlockBackend is being quiesced.
> +     *
> +     * This initial test does not have to be perfect: a race might
> +     * cause an extra I/O to be queued, but sooner or later a nonzero
> +     * quiesce_counter will be observed.

This is true in the initial drain phase while we're still polling. But
generally this is not only about avoiding a continuous stream of
requests, but about making sure that absolutely no new requests come in
while a node is drained.

> +     */
> +    if (!blk->disable_request_queuing && qatomic_read(&blk->quiesce_counter)) {

So if no other requests were pending and we didn't even call aio_poll()
because the AIO_WAIT_WHILE() condition was false from the start, could
it be that bdrv_drained_begin() has already returned on the thread that
drains, but another thread still sees the old value here?

Starting a new request after bdrv_drained_begin() has returned would be
a bug.

Kevin