Re: [PATCH] epoll: add exclusive wakeups flag

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Jason,

On 03/15/2016 08:32 AM, Jason Baron wrote:
> 
> 
> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote:
>> [Restoring CC, which I see I accidentally dropped, one iteration back.]

[...]

>>>>               values in events yield an error.  EPOLLEXCLUSIVE may  be
>>>>               used  only  in  an  EPOLL_CTL_ADD operation; attempts to
>>>>               employ  it  with  EPOLL_CTL_MOD  yield  an  error.    If
>>>>               EPOLLEXCLUSIVE has set using epoll_ctl(2), then a subse‐
>>>>               quent EPOLL_CTL_MOD on the same epfd, fd pair yields  an
>> b>>               error.  An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in
>>>>               events and specifies the target file descriptor fd as an
>>>>               epoll  instance will likewise fail.  The error in all of
>>>>               these cases is EINVAL.
>>>>
>>>>    ERRORS
>>>>        EINVAL An invalid event type was specified along with  EPOLLEX‐
>>>>               CLUSIVE in events.
>>>>
>>>>        EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.
>>>>
>>>>        EINVAL op  was  EPOLL_CTL_MOD  and  the EPOLLEXCLUSIVE flag has
>>>>               previously been applied to this epfd, fd pair.
>>>>
>>>>        EINVAL EPOLLEXCLUSIVE was specified in event and fd  is  refers
>>>>               to an epoll instance.
>>
>> Returning to the second sentence in this description:
>>
>>               When a wakeup event occurs and multiple epoll file descrip‐
>>               tors are attached to the same target file using EPOLLEXCLU‐
>>               SIVE, one or  more  of  the  epoll  file  descriptors  will
>>               receive  an  event with epoll_wait(2).
>>
>> There is a point that is unclear to me: what does "target file" refer to?
>> Is it an open file description (aka open file table entry) or an inode?
>> I suspect the former, but it was not clear in your original text.
>>
> 
> So from epoll's perspective, the wakeups are associated with a 'wait
> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via
> file->poll()) results in adding to the same 'wait queue' then we will
> get 'exclusive' wakeup behavior.
> 
> So in general, I think the answer here is that its associated with the
> inode (I coudn't say with 100% certainty without really looking at all
> file->poll() implementations). Certainly, with the 'FIFO' example below,
> the two scenarios will have the same behavior with respect to
> EPOLLEXCLUSIVE.

So, in both scenarios, *one or more* processes will get a wakeup?
(I'll try to add something to the text to clarify the detail we're 
discussing.)

> Also, the 'non-exclusive' mode would be subject to the same question of
> which wait queue is the epfd is associated with...

I'm not sure of the point you are trying to make here?

Cheers,

Michael


>> To make this point even clearer, here are two scenarios I'm thinking of.
>> In each case, we're talking of monitoring the read end of a FIFO.
>>
>> ===
>>
>> Scenario 1:
>>
>> We have three processes each of which
>> 1. Creates an epoll instance
>> 2. Opens the read end of the FIFO
>> 3. Adds the read end of the FIFO to the epoll instance, specifying
>>    EPOLLEXCLUSIVE
>>
>> When input becomes available on the FIFO, how many processes
>> get a wakeup?
>>
>> ===
>>
>> Scenario 3
>>
>> A parent process opens the read end of a FIFO and then calls
>> fork() three times to create three children. Each child then:
>>
>> 1. Creates an epoll instance
>> 2. Adds the read end of the FIFO to the epoll instance, specifying
>> EPOLLEXCLUSIVE
>>
>> When input becomes available on the FIFO, how many processes
>> get a wakeup?
>>
>> ===
>>
>> Cheers,
>>
>> Michael
>>
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/