linux-kernel - Re: [patch 2/5] signalfd v2

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Date:	Thu, 8 Mar 2007 08:22:57 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
cc:	"David M. Lloyd" <dmlloyd@...rg.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 2/5] signalfd v2 - signalfd core ...

On Thu, 8 Mar 2007, Davide Libenzi wrote:
> 
> This patch, if you get a POLLIN, you have a signal to read for sure (well, 
> unless you another thread/task reads it before you - but that's just 
> somthing you have to take care). There is not explicit check for 
> O_NONBLOCK now, but a zero timeout would do exactly the same thing.

You missed David's worry, I think.

Not only is POLLIN potentially an edge event (depending on the interface 
you use to fetch it), but even as a level-triggered one you generally want 
to read as much as possible per POLLIN event, and go back to the event 
loop only when you get EAGAIN.

So that's in addition to the read/signal race with other 
threads/processes.

You solved it by having a separate system call, but since it's a regular 
file descriptor, why have a new system call at all, and not just make it 
be a "read()"? In which case you definitely want O_NONBLOCK support.

The UNIX philosophy is often quoted as "everything is a file", but that 
really means "everything is a stream of bytes".

In Windows, you have 15 different versions of "read()" with sockets and 
files and pipes all having strange special cases and special system calls. 
That's not the UNIX way. It should be just a "read()", and then people can 
use general libraries and treat all sources the same.

For example, the main select/poll/epoll loop may be the one doing all the 
reading, and then pass off "full buffers only" to the individual per-fd 
"action routines". And that kind of model really very fundamentally wants 
an fd to be an fd to be an fd - not "some file descriptors need 
'read_from_sigfd()', and some file descriptors need 'read()', and some 
file descriptors need 'recvmsg()'" etc.

So I think you should get rid of signalfd_dequeue(), and just replace it 
with a "read()" function.

		Linus
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