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Date:	Thu, 8 Mar 2007 11:21:18 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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, Linus Torvalds wrote:

> 
> 
> 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)

I can do that, no problem. But isn't it better to recognize that this kind 
of data just can't be shipped through a non compat-able function?
Like, for example, the current trend to say "just use u64 for a pointer, 
it'll be fine". I remeber, a long time ago when the i386 architecture came 
out, to say "Wow! 4GB is gonna last *forever*!", let's use u32 for 
pointers. Well, forever is almost here in my watches. And all the 
userspace code using APIs assuming to cleanly store a pointer in a u32 
will have to be re-factored.
So, to cut it short, I can do the pseudo-siginfo read(2), but I don't 
like it too much (little, actually). The siginfo, as bad as it is, is a 
standard used in many POSIX APIs (hence even in kernel), and IMO if we 
want to send that back, a struct siginfo should be.
No?



- Davide


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