linux-kernel - Re: LinuxPPS & spinlocks

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Message-ID: <a781481a0707271347w53dd9755j87e7762cfe953777@mail.gmail.com>
Date:	Sat, 28 Jul 2007 02:17:24 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Rodolfo Giometti" <giometti@...eenne.com>
Cc:	"Chris Friesen" <cfriesen@...tel.com>,
	"David Woodhouse" <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: LinuxPPS & spinlocks

Hi Rodolfo,

On 7/28/07, Rodolfo Giometti <giometti@...eenne.com> wrote:
> On Fri, Jul 27, 2007 at 01:40:14PM -0600, Chris Friesen wrote:
> >
> > My point is that the lock should be used to protect specific data. Thus, it
> > would be more correct to say, "spinlock foo is taken because
> > pps_register_source() accesses variable bar".
> >
> > That way, if someone else wants to access "bar", they know that they need
> > to take lock "foo".
>
> Ah, ok! I see. :)

I only glanced through the code, so could be wrong, but I noticed that
the only global / shared data you have in there is a global "pps_source"
array of pps_s structs. That's accessed / modified from the various
syscalls introduced in the API exported to userspace, as well as the
register/unregister/pps_event API exported to in-kernel client subsystems,
yes? So it looks like you need to introduce proper locking for it, simply
type-qualifying it as "volatile" is not enough.

However, I think you've introduced two locks for it. The syscalls (that
run in process context, obviously) seem to use a pps_mutex and
pps_event() seems to be using the pps_lock spinlock (because that
gets executed from interrupt context) -- and from the looks of it, the
register/unregister functions are using /both/ the mutex and spinlock (!)

This isn't quite right, (in fact there's nothing to protect pps_event from
racing against a syscall), so you should use *only* the spinlock for
synchronization -- the spin_lock_irqsave/restore() variants, in fact.

[ Also, have you considered making pps_source a list and not an array?
It'll help you lose a whole lot of MAX_SOURCES, pps_is_allocated, etc
kind of gymnastics in there, and you _can_ return a pointer to the
corresponding pps source struct from the register() function to the in-kernel
users, so that way you get to retain the O(1) access to the corresponding
source when a client calls into pps_event(), similar to how you're using the
array index presently. ]

I also noticed code like (from pps_event):

+	/* Try to grab the lock, if not we prefere loose the event... */
+	if (!spin_trylock(&pps_lock))
+		return;

which looks worrisome and unnecessary. That spinlock looks to be of
fine enough granularity to me, do you think there'd be any contention
on it? I /think/ you can simply make that a spin_lock().

Overall the code looks simple / straightforward enough to me (except for
the parport / uart stuff that I have no clue about), and I'll also read up on
the relevant RFC for this and would hopefully try and give you a more
meaningful review over the weekend.

Thanks,
Satyam
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