Re: [PATCH] epoll: add exclusive wakeups flag

Hi Jason,

On 03/15/2016 08:32 AM, Jason Baron wrote:

On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote:

quoted

[Restoring CC, which I see I accidentally dropped, one iteration back.]

[...]

quoted

quoted

quoted

              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

quoted

quoted

              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

quoted

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/
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