Re: [PATCH 10/15] block-backend: always wait for drain before starting operation

From: Kevin Wolf <kwolf@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, eesposit@redhat.com
Subject: Re: [PATCH 10/15] block-backend: always wait for drain before starting operation
Date: Mon, 16 Jan 2023 17:48:59 +0100	[thread overview]
Message-ID: <Y8V/+zAmvRH/CyXA@redhat.com> (raw)
In-Reply-To: <20221212125920.248567-11-pbonzini@redhat.com>

Am 12.12.2022 um 13:59 hat Paolo Bonzini geschrieben:
> All I/O operations call blk_wait_while_drained() immediately after
> blk_inc_in_flight(), except for blk_abort_aio_request() where it
> does not hurt to add such a call.  Merge the two functions into one,
> and add a note about a disturbing lack of thread-safety that will
> be fixed shortly.
> 
> While at it, make the quiesce_counter check a loop.  While the check
> does not have to be "perfect", i.e. it only matters that an endless
> stream of I/Os is stopped sooner or later, it is more logical to check
> the counter repeatedly until it is zero.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/block-backend.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index fe42d53d655d..627d491d4155 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -1270,18 +1270,6 @@ static int blk_check_byte_request(BlockBackend *blk, int64_t offset,
>      return 0;
>  }
>  
> -/* To be called between exactly one pair of blk_inc/dec_in_flight() */
> -static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
> -{
> -    assert(blk->in_flight > 0);
> -
> -    if (blk->quiesce_counter && !blk->disable_request_queuing) {
> -        blk_dec_in_flight(blk);
> -        qemu_co_queue_wait(&blk->queued_requests, NULL);
> -        blk_inc_in_flight(blk);
> -    }
> -}
> -
>  /* To be called between exactly one pair of blk_inc/dec_in_flight() */
>  static int coroutine_fn
>  blk_co_do_preadv_part(BlockBackend *blk, int64_t offset, int64_t bytes,
> @@ -1334,7 +1322,6 @@ int coroutine_fn blk_co_preadv(BlockBackend *blk, int64_t offset,
>      IO_OR_GS_CODE();
>  
>      blk_inc_in_flight(blk);
> -    blk_wait_while_drained(blk);
>      ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, 0, flags);
>      blk_dec_in_flight(blk);
>  
> @@ -1349,7 +1336,6 @@ int coroutine_fn blk_co_preadv_part(BlockBackend *blk, int64_t offset,
>      IO_OR_GS_CODE();
>  
>      blk_inc_in_flight(blk);
> -    blk_wait_while_drained(blk);
>      ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, qiov_offset, flags);
>      blk_dec_in_flight(blk);
>  
> @@ -1401,7 +1387,6 @@ int coroutine_fn blk_co_pwritev_part(BlockBackend *blk, int64_t offset,
>      IO_OR_GS_CODE();
>  
>      blk_inc_in_flight(blk);
> -    blk_wait_while_drained(blk);
>      ret = blk_co_do_pwritev_part(blk, offset, bytes, qiov, qiov_offset, flags);
>      blk_dec_in_flight(blk);
>  
> @@ -1466,6 +1451,14 @@ 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! */
> +        while (blk->quiesce_counter) {
> +            qatomic_dec(&blk->in_flight);
> +            qemu_co_queue_wait(&blk->queued_requests, NULL);

blk_inc_in_flight() must be a coroutine_fn now.

blk_abort_aio_request() and blk_aio_prwv() aren't, but still call it.

> +            qatomic_inc(&blk->in_flight);
> +        }
> +    }
>  }

Kevin