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Message-ID: <56E6F941.9040307@gmail.com>
Date: Tue, 15 Mar 2016 06:47:45 +1300
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Jason Baron <jbaron@...mai.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: mtk.manpages@...il.com, mingo@...nel.org, peterz@...radead.org,
viro@....linux.org.uk, normalperson@...t.net, m@...odev.com,
corbet@....net, luto@...capital.net, torvalds@...ux-foundation.org,
hagen@...u.net, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] epoll: add exclusive wakeups flag
[Restoring CC, which I see I accidentally dropped, one iteration back.]
Hi Jason,
Thanks for the review. I've tweaked one piece to respond to your
feedback. But I also have another new question below.
On 03/15/2016 03:55 AM, Jason Baron wrote:
> On 03/11/2016 06:25 PM, Michael Kerrisk (man-pages) wrote:
>> On 03/11/2016 09:51 PM, Jason Baron wrote:
>>> On 03/11/2016 03:30 PM, Michael Kerrisk (man-pages) wrote:
[...]
> Hi Michael,
>
> Looks good. One comment below.
>
> Thanks,
>
>> EPOLLEXCLUSIVE (since Linux 4.5)
>> Sets an exclusive wakeup mode for the epoll file
>> descriptor that is being attached to the target file
>> descriptor, fd. When a wakeup event occurs and multiple
>> epoll file descriptors are attached to the same target
>> file using EPOLLEXCLUSIVE, one or more of the epoll file
>> descriptors will receive an event with epoll_wait(2).
>> The default in this scenario (when EPOLLEXCLUSIVE is not
>> set) is for all epoll file descriptors to receive an
>> event. EPOLLEXCLUSIVE is thus useful for avoiding thun‐
>> dering herd problems in certain scenarios.
>>
>> If the same file descriptor is in multiple epoll
>> instances, some with the EPOLLEXCLUSIVE flag, and others
>> without, then events will provided to all epoll
>> instances that did not specify EPOLLEXCLUSIVE, and at
>> least one of the epoll instances that did specify
>> EPOLLEXCLUSIVE.
>>
>> The following values may be specified in conjunction
>> with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and
>> EPOLLET. EPOLLHUP and EPOLLERR can also be specified,
>> but are ignored (as usual). Attempts to specify other
>
> I'm not sure 'ignored' is the right wording here. 'EPOLLHUP' and
> 'EPOLERR' are always included in the set of events when something is
> added as EPOLLEXCLUSIVE. This is consistent with the non-EPOLLEXCLUSIVE
> add case.
Yes.
> So 'EPOLLHUP' and 'EPOLERR' may be specified but will be
> included in the set of events on an add, whether they are specified or not.
Yes. I understand your discomfort with the work "ignored", but the
problem was that, because it made special mention of EPOLLHUP and EPOLLERR,
your proposed text made it sound as though EPOLLEXCLUSIVE somehow was
special with respect to these two flags. I wanted to clarify that it is not.
How about this:
The following values may be specified in conjunction
with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and
EPOLLET. EPOLLHUP and EPOLLERR can also be specified,
but this is not required: as usual, these events are
always reported if they occur, regardless of whether
they are specified in events.
?
>> 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.
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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