Re: [PATCH] aio-posix: fix spurious ->poll_ready() callbacks in main loop

[Stefan Hajnoczi]

On Wed, Feb 23, 2022 at 03:57:03PM +0000, Stefan Hajnoczi wrote:
> When ->poll() succeeds the AioHandler is placed on the ready list with
> revents set to the magic value 0. This magic value causes
> aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
> for G_IO_IN or ->io_write() for G_IO_OUT.
> 
> This magic value 0 hack works for the IOThread where AioHandlers are
> placed on ->ready_list and processed by aio_dispatch_ready_handlers().
> It does not work for the main loop where all AioHandlers are processed
> by aio_dispatch_handlers(), even those that are not ready and have a
> revents value of 0.
> 
> As a result the main loop invokes ->poll_ready() on AioHandlers that are
> not ready. These spurious ->poll_ready() calls waste CPU cycles and
> could lead to crashes if the code assumes ->poll() must have succeeded
> before ->poll_ready() is called (a reasonable asumption but I haven't
> seen it in practice).
> 
> Stop using revents to track whether ->poll_ready() will be called on an
> AioHandler. Introduce a separate AioHandler->poll_ready field instead.
> This eliminates spurious ->poll_ready() calls in the main loop.
> 
> Fixes: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("aio-posix: split poll check 
> from ready handler")
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  util/aio-posix.h |  1 +
>  util/aio-posix.c | 32 ++++++++++++++++++--------------
>  2 files changed, 19 insertions(+), 14 deletions(-)

Applied to my block tree:
https://gitlab.com/stefanha/qemu/commits/block

Stefan

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