Re: [libvirt] [PATCH v2 3/3] util: event loop: document another reason to defer deletion
On 11/14/2016 09:27 AM, Nikolay Shirokovskiy wrote:
So the above patches are pushed now. What about the last one?
I've elaborated
on its meaning in sibling letter.
Nikolay
You can always retry with a new patch that adds what you think will be
helpful. Keep in mind what I wrote, what you wrote, and propose the
adjustment.
I agree with your feeling that perhaps the code isn't as self
documenting as perhaps originally felt, but in order to alter the
descriptions (comments) - I think the new comments should be more
helpful. Like I pointed out in my response - I was having difficulty
figuring out what was being noted by just reading the new comments.
After reading the cover, other code, Dan's response to your original
series, I started to piece together things which is what I posted. I
think at one time I was going to note perhaps adjusting the docs as
well, e.g. http://libvirt.org/internals/eventloop.html
John
On 28.10.2016 00:58, John Ferlan wrote:
>
>
> On 10/04/2016 10:27 AM, Nikolay Shirokovskiy wrote:
>> This is why we should not free callback object synchronously
>> upon removing handle or timeout. Imagine:
>>
>> 1. loop thread, drops the lock and is about to call event callback
>> 2. another thread, enters remove handle and frees callback object
>> 3. loop thread, enters event callback, uses callback object BOOOM
>> ---
>> src/util/vireventpoll.c | 7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>
>
> I'm having difficulty trying to decipher the point you're trying to make
> in the context of the existing comments, the previous upstream series,
> and the cover letter explanation.
>
> While not explicitly stated for each, the 'flag' that's set is
> 'deleted'. Once the 'deleted' flag is set, it's expected that the
object
> will be reaped later when it's safer to do so (such as
> virEventPollCleanupTimeouts and virEventPollCleanupHandles) via the 'ff'
> passed during virEventAddHandle and virEventAddTimeout.
>
> I'm going to pass on pushing this one.
>
> John
>
>
>> diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c
>> index 81ecab4..802b679 100644
>> --- a/src/util/vireventpoll.c
>> +++ b/src/util/vireventpoll.c
>> @@ -176,6 +176,10 @@ void virEventPollUpdateHandle(int watch, int events)
>> * Unregister a callback from a file handle
>> * NB, it *must* be safe to call this from within a callback
>> * For this reason we only ever set a flag in the existing list.
>> + * Another reason is that as we drop the lock upon event callback
>> + * invocation we can't free callback object if we called
>> + * from a thread then loop thread.
>> + *
>> * Actual deletion will be done out-of-band
>> */
>> int virEventPollRemoveHandle(int watch)
>> @@ -295,6 +299,9 @@ void virEventPollUpdateTimeout(int timer, int frequency)
>> * Unregister a callback for a timer
>> * NB, it *must* be safe to call this from within a callback
>> * For this reason we only ever set a flag in the existing list.
>> + * Another reason is that as we drop the lock upon event callback
>> + * invocation we can't free callback object if we called
>> + * from a thread then loop thread.
>> * Actual deletion will be done out-of-band
>> */
>> int virEventPollRemoveTimeout(int timer)
>>