linux-kernel - Re: WaitForMultipleObjects/etc. In Kernel

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Date:	Tue, 28 Jan 2014 10:04:02 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Network Nut <sillystack@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: WaitForMultipleObjects/etc. In Kernel

Network Nut wrote:
> 5. I can simulate system-global named mutex using shared-memory for
> underlying state of mutex (POCO NamedMutex)
> 6. I can get named semaphore using POSIX sem_create
>
> It seems that the remaining problem is to get named mutex and named
> semaphore to be accessible by file-descriptor.

Forget about the POSIX stuff.  You can implement a mutex by using
an eventfd that has the value 1 in the unlocked state; a read locks,
a write of 1 unlocks.  You can implement a semaphore by using an eventfd
with EFD_SEMAPHORE.

And I should note that it is a common pattern to start a bunch of other
processes from a helper process; this not only allows inheriting file
descriptors, but also makes monitoring these processes easier from the
parent.  Do all of your processes really need to be started
independently?

>>> "The timeout argument specifies the minimum number of milliseconds
>>>        that epoll_wait() will block."
>>>
>>> I think the word "minimum" should be "maximum".
>>
>> This sentence was copied from the poll(2) man page, where the previous
>> sentence says "poll() blocks until one of the events occurs".  So the word
>> "block" implies that no event has occured.
>
> So that means that it is a bug, right? Or?

The purpose of a man page is certainly not to confuse its readers.  I'd
guess the maintainer would be happy about an improved wording.

Regards,
Clemens
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