Re: LinuxPPS & spinlocks

[Rodolfo Giometti <giometti@enneenne.com>]

On Sat, Jul 28, 2007 at 02:17:24AM +0530, Satyam Sharma wrote:
> 
> 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 is right.

> 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.

We can't use the spin_lock_irqsave/restore() variants since PPS
sources cannot manage IRQ enable/disable. For instance, the serial
source doesn't manage IRQs directly but just uses it to record PPS
events. The serial driver manages the IRQ enable/disable, not the PPS
source which only uses the IRQ handler to records events.

About using both mutex and spinlock I did it since (I think) I should
protect syscalls from each others and from pps_register/unregister(),
and pps_event() against pps_register/unregister().

> [ 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().

This is due the fact I cannot manage IRQ enable/disable.

> 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 a lot for your help!

Ciao,

Rodolfo

-- 

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