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

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/11/2017 14:47, Kevin Wolf wrote:
> 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.

Makes sense!  Since co_queue_wakeup is used on *yield*, maybe the order
should be: ctx, scheduled, co_queue_next, co_queue_wakeup,
co_scheduled_next.

Thanks,

Paolo