spin_lock and scheduler confusion

[Tirtha Ghosh]

NMI has greater priority over spinlock, cause this is non maskable and NMI
watchdog can be used for debugging spinlock deadlocks
(CONFIG_DEBUG_SPINLOCK).

So we will hit NMI watchdog even if spinlock is acquired.



On Fri, Jan 7, 2011 at 11:57 AM, Tayade, Nilesh
<Nilesh.Tayade at netscout.com>wrote:

> Hi,
>
> > -----Original Message-----
> > From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-
> > bounces at kernelnewbies.org] On Behalf Of Dave Hylands
> > Sent: Friday, January 07, 2011 10:59 AM
> > To: Viral Mehta
> > Cc: kernelnewbies at kernelnewbies.org
> > Subject: Re: spin_lock and scheduler confusion
> >
> > Hi Viral,
> >
> > On Wed, Jan 5, 2011 at 2:23 PM, Viral Mehta
> > <Viral.Mehta at lntinfotech.com> wrote:
> > >
> > > Hi ,
> > >
> > > I need your help to solve below confusion.
> > >
> [...]
> >
> > Note that you can't sleep while you hold a spinlock. You're not
> > allowed to perform any type of blocking operations. If you're holding
> > the spinlock for any significant length of time, then you're using the
> > wrong design.
> >
> > >     spin_lock_irqrestore();
> > > 3. One of the CPU core tries to execute this code and so acquires the
> > lock.
> > > 4. Now, second core is also goes to execute same piece of code and so
> > will
> [...]
> >
> > Not while it's holding the spinlock or waiting for the spinlock.
> >
> > > Ever if timeslice is over for the current task ?
> >
> > The time tick interrupt is what determines when the timeslice is over.
> > Since you have interrupts disabled, the timer interrupt can't happen.
> >
> > > What if scheduler code is running on CPU core-3 and sees that
> > > timeslice for task running on CPU core-2 has expired ?
> >
> > Each core only considers the timeslices for its own core.
> >
> > > I guess timeslice expire case is not as same as preemption. Or may be
> > I am
> > > terribly wrong.
> >
> > You shouldn't be holding  a spinlock for periods of time approaching
> > the length of a timeslice. The timer interrupt is what determines the
> > end of a timeslice. No timer interrupt, no end of a timeslice.
> > Preemption is also triggered by the timer interrupt, or by releasing a
> > resource that a higher priority task is waiting for.
>
> May be my understanding is incorrect, but wouldn't we hit the NMI watchdog
> here(assuming we are running on x86/x86_64)?
> We have a system lockup for long time.
> http://lxr.linux.no/#linux+v2.6.37/Documentation/nmi_watchdog.txt
>
> Could someone please clarify?
> >
> > Dave Hylands
>
> --
> Thanks,
> Nilesh
>
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