On Tue, May 19, 2009 at 10:52 PM, Mulyadi Santosa <[email protected]
> wrote:
> Hi...
>
> Sorry if my earlier post confused some people. What I wanted to say
> was actually that spinlock is SMP safe, however, in multicore it makes
> code running on a cpu might spin to wait for other code running on
> different cpu.
If the data is unique to each processor, there may be no need to
protect it with a lock because only that one processor can access the data.
In this case we need to use preempt_disable() and preempt_enable() to
protect data from concurrent access by multiple processes.
Thanks
Shyam
>
>
> so, to avoid this, perhaps the original poster could gain better
> scalability by adopting per CPU data structure. Therefore, spin lock
> usage is reduced only on cases e.g data must be global across CPU or
> blocking wait is not an option
>
> regards,
>
> Mulyadi.
>
>
> On 5/19/09, Shyam Burkule <[email protected]> wrote:
> > All,
> >
> > On Tue, May 19, 2009 at 3:33 AM, raz ben yehuda <[email protected]>
> wrote:
> >
> >>
> >> On Tue, 2009-05-19 at 00:49 +0700, Mulyadi Santosa wrote:
> >> > On Tue, May 19, 2009 at 12:33 AM, Ole Loots <[email protected]>
> wrote:
> >> > > Hello to the list,
> >> > >
> >> > > I got a question about spinlocks. Here is some pseudo-code:
> >> > >
> >> > > my_external_int_handler(...)
> >> > > {
> >> > > spin_lock(&my_lock);
> >> > > // do things...
> >> > > spin_unlock(&my_lock);
> >> > > }
> >> > >
> >> > > my_ioctl_handler(ioctl_value)
> >> > > {
> >> > > switch(ioctl_value)
> >> > > {
> >> > > case xy:
> >> > > spin_lock_irqsave(&my_lock, flags);
> >> > > // do stuff
> >> > > spin_unlock_irqsave(&my_lock, flags);
> >> > > break;
> >> > > }
> >> > > }
> >> > >
> >> > > I just wan't to ensure that the interrupt is finished before I
> handle
> >> the
> >> > > IOCTL request, so that I'm not running into a race condition that
> >> > > would
> >> a
> >> > > affect an ring buffer.
> >> > > But what happens when an interrupt signal is triggered at the
> >> > > external
> >> > > interrupt pin, does spin_lock_irqsave que the interrupt? Or does it
> >> dismiss
> >> > > the interrupt? Does spinlock_irqsave mean I would miss an interrupt?
> >> > > If
> >> so,
> >> > > spinlock won't be the right thing to do...
> >> > >
> >> > > What I need is something like:
> >> > > while(interrupt_working){ sleep(); }
> >> > >
> >> > > How to do right?
> >> >
> >> > i think spinlock_irqsave means it disables the interrupt pins
> >> > temporarily but not nullifying the queued interrupts. Once it is
> >> > enabled again, it would fire the handler again.
> >> >
> >> > However, IIRC too, spin_lock_irqsave is disabling per CPU interrupt
> >> > line. So if you run your code in SMP or multicore, there is a chance
> >> please fix me if i am wrong.
> >> what spinlock_irqsave does is:
> >> 1. saves current of local interrupts.
> >> 2. disables local interrupts
> >> 3. acquires a lock
> >> any other context tries to to access the protected data will spin. else,
> >> how would you protect the data in SMP ?
> >
> >
> >
> > Spinlock are meant to write SMP safe routines. On UP, using spinlock get
> > compiled to disable/enable of kernel preemption.
> >
> > spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
> > unsigned long flags;
> >
> >
> > spin_lock_irqsave(&xxx_lock, flags);
> > /* Critical Section*/
> > spin_unlock_irqrestore(&xxx_lock, flags);
> >
> > Above code is always safe. It disable interrupts *locally* but the
> > spinlock guarantee
> > the global lock.
> >
> >
> >
> >
> >
> >
> >
> >>
> >> > of both interrupt and ioctl handlers try to access the data. Although
> >> > it is expected, perhaps you could consider using per CPU data ?
> >> >
> >> > regards,
> >> >
> >> > Mulyadi.
> >> >
> >> > --
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> >> >
> >>
> >>
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> >>
> >
>
