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Re: [PATCH v2 2/2] mirror: Wait only for in-flight operations

From: Max Reitz
Subject: Re: [PATCH v2 2/2] mirror: Wait only for in-flight operations
Date: Thu, 26 Mar 2020 19:27:14 +0100
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On 26.03.20 16:36, Kevin Wolf wrote:
> mirror_wait_for_free_in_flight_slot() just picks a random operation to
> wait for. However, a MirrorOp is already in s->ops_in_flight when
> mirror_co_read() waits for free slots, so if not enough slots are
> immediately available, an operation can end up waiting for itself, or
> two or more operations can wait for each other to complete, which
> results in a hang.
> Fix this by adding a flag to MirrorOp that tells us if the request is
> already in flight (and therefore occupies slots that it will later
> free), and picking only such operations for waiting.
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1794692
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/mirror.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> diff --git a/block/mirror.c b/block/mirror.c
> index 393131b135..88414d1653 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c


> @@ -1318,6 +1324,7 @@ static MirrorOp *coroutine_fn 
> active_write_prepare(MirrorBlockJob *s,
>          .offset             = offset,
>          .bytes              = bytes,
>          .is_active_write    = true,
> +        .is_in_flight       = true,
>      };
>      qemu_co_queue_init(&op->waiting_requests);
>      QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);

There is a mirror_wait_on_conflicts() call after this.  I was a bit
worried about dependencies there.  But I don’t think there’s any
problem, because mirror_wait_for_any_operation() is only called by:

(1) mirror_wait_for_free_in_flight_slot(), which makes it look for
non-active operations only, and

(2) mirror_run(), which specifically waits for all active operations to
settle, so it makes sense to wait for all of them, even when they are
still doing their own dependency-waiting.

But still, I’m not sure whether this is conceptually the best thing to
do.  I think what we actually want is for
mirror_wait_for_free_in_flight_slot() to only wait for in-flight
operations; but the call in mirror_run() that waits for active-mirror
operations wants to wait for all active-mirror operations, not just the
ones that are in flight.

So I think conceptually it would make more sense to set is_in_flight
only after mirror_wait_on_conflicts(), and ensure that the
mirror_wait_for_any_operation() call from mirror_run() ignores
is_in_flight.  E.g. by having another parameter “bool in_flight” for
mirror_wait_for_any_operation() that chooses whether to check
is_in_flight or whether to ignore it.

In practice, @in_flight would always be the same as @active, but they
are different things.  But that would mean we would always ignore
is_in_flight for active-mirror operations.

In practice, there’s no difference to what this patch does, i.e. to just
let active-mirror operations have is_in_flight to be always true and let
mirror_wait_for_any_operation() check is_in_flight unconditionally.

So I don’t know.  Maybe this is a start:

Functionally-reviewed-by: Max Reitz <address@hidden>


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