linux-kernel - Re: [PATCH 1/1] eventfd: implementation of EFD

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Message-ID: <5113FCA7.4020207@mit.edu>
Date:	Thu, 07 Feb 2013 11:12:39 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Martin Sustrik <sustrik@...bpm.com>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sha Zhengju <handai.szj@...bao.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] eventfd: implementation of EFD_MASK flag

On 02/06/2013 10:41 PM, Martin Sustrik wrote:
> When implementing network protocols in user space, one has to implement
> fake user-space file descriptors to represent the sockets for the protocol.
> 
> While all the BSD socket API functionality for such descriptors may be faked as
> well (myproto_send(), myproto_recv() etc.) this approach doesn't work for
> polling  (select, poll, epoll). For polling, real system-level file descriptor
> is needed.
> 
> In theory, eventfd may be used for this purpose, except that it is well suited
> only for signaling POLLIN. With some hacking it can be also used to signal
> POLLOUT and POLLERR, however:
> 
> I.  There's no way to signal POLLPRI, POLLHUP etc.
> II. There's no way to signal arbitraty combination of POLL* flags. Most notably,
>     !POLLIN & !POLLOUT, which is a perfectly valid combination for a network
>     protocol (rx buffer is empty and tx buffer is full), cannot be signaled
>     using current implementation of eventfd.
> 
> This patch implements new EFD_MASK flag which attempts to solve this problem.
> 
> Additionally, when implementing network protocols in user space, there's a
> need to associate user-space state with the each "socket". If eventfd object is
> used as a reference to the socket, it should be possible to associate an opaque
> pointer to user-space data with it.
> 
> The semantics of EFD_MASK are as follows:
> 
> eventfd(2):
> 
> If eventfd is created with EFD_MASK flag set, it is initialised in such a way
> as to signal no events on the file descriptor when it is polled on. 'initval'
> argument is ignored.
> 
> write(2):
> 
> User is allowed to write only buffers containing the following structure:
> 
> struct efd_mask {
>   short events;
>   void *ptr;
> };

IMO that should be u64 ptr to avoid compat problems.

> 
> The value of 'events' should be any combination of event flags as defined by
> poll(2) function (POLLIN, POLLOUT, POLLERR, POLLHUP etc.) Specified events will
> be signaled when polling (select, poll, epoll) on the eventfd is done later on.
> 'ptr' is an opaque pointer that is not interpreted by eventfd object.

How does this interact with EPOLLET?

--Andy
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