Re: [Qemu-devel] [PATCH v2 for-2.11 2/4] coroutine: abort if we try to schedule or enter a pending coroutine

[Kevin Wolf <kwolf@redhat.com>]

Am 21.11.2017 um 03:23 hat Jeff Cody geschrieben:
> The previous patch fixed a race condition, in which there were
> coroutines being executing doubly, or after coroutine deletion.
> 
> We can detect common scenarios when this happens, and print an error
> message and abort before we corrupt memory / data, or segfault.
> 
> This patch will abort if an attempt to enter a coroutine is made while
> it is currently pending execution, either in a specific AioContext bh,
> or pending execution via a timer.  It will also abort if a coroutine
> is scheduled, before a prior scheduled run has occured.
> 
> We cannot rely on the existing co->caller check for recursive re-entry
> to catch this, as the coroutine may run and exit with
> COROUTINE_TERMINATE before the scheduled coroutine executes.
> 
> (This is the scenario that was occuring and fixed in the previous
> patch).
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  include/qemu/coroutine_int.h |  6 ++++++
>  util/async.c                 | 11 +++++++++++
>  util/qemu-coroutine-sleep.c  | 11 +++++++++++
>  util/qemu-coroutine.c        | 11 +++++++++++
>  4 files changed, 39 insertions(+)
> 
> diff --git a/include/qemu/coroutine_int.h b/include/qemu/coroutine_int.h
> index cb98892..56e4c48 100644
> --- a/include/qemu/coroutine_int.h
> +++ b/include/qemu/coroutine_int.h
> @@ -53,6 +53,12 @@ struct Coroutine {
>  
>      /* Only used when the coroutine has yielded.  */
>      AioContext *ctx;
> +
> +    /* Used to catch and abort on illegal co-routine entry.
> +     * Will contain the name of the function that had first
> +     * scheduled the coroutine. */
> +    const char *scheduled;

Not sure if it makes any difference in practice, but I just want to
mention that the new field is right after a cacheline boundary and
the only field that is used in qemu_aio_coroutine_enter() and accesses
this second cacheline.

I'm not paying much attention to this kind of thing in most contexts,
but entering a coroutine is a hot path that we want to be fast, so maybe
it's worth having a second look.

Kevin