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

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Date:	Thu, 8 Mar 2007 09:15:57 -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:
> 
> The reason for the special function, was not to provide a non-blocking 
> behaviour with zero timeout (that just a side effect), but to read the 
> siginfo. I was all about using read(2) (and v1 used it), but when you have 
> to transfer complex structures over it, it becomes hell. How do you 
> cleanly compat over a f_op->read callback for example?

I agree that it gets a bit "interesting", and one option might be that the 
"read()" interface just gets the signal number and the minimal siginfo 
information, which is, after all, what 99% of all apps actually care
about.

But "siginfo_t" is really a *horrible* structure. Nobody sane should ever 
use siginfo_t, and the designer of that thing was so high on LSD that it's 
not even funny. Re-using fields in a union? Values that depend on other 
bits in the thing in random manners? 

In other words, I bet that we could just make it a *lot* better by making 
the read structure be:

 - 16 4-byte fields (fixed 64-byte packet), where each field is an 
   uint32_t (we could even do it in network byte order if we care and if 
   you want to just pipe the information from one machine to another, but 
   that sounds a bit excessive ;)

 - Just put the fields people actually use at fixed offsets: si_signo, 
   si_errno, si_pid, si_uid, si_band, si_fd.

 - that still leaves room for the other cases if anybody ever wants them 
   (but I doubt it - things like si_addr are really only useful for 
   synchronous signals that are actually done as *signals*, since you 
   cannot defer a SIGBUS/SIGSEGV/SIGILL *anyway*).

So I bet 99% of users actually just want si_signo, while some small subset 
might want the SIGCHLD info and some of the special cases (eg we migth 
want to add si_addr as a 64-bit thing just because the USB stack sends a 
SI_ASYNCIO thing for completed URB's, so a libusb might want it, but 
that's probably the only such user).

And it would be *cleaner* than the mess that is siginfo_t..

(I realize that siginfo_t is ugly because it built up over time, using the 
same buffer for many different things. I'm just saying that we can 
probably do better by *not* using it, and just laying things out in a 
cleaner manner to begin with, which also solves any compatibility issues)

		Linus
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