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
[off-list ref] wrote:

quoted

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.

quoted

??? spin_lock_irqrestore();
3. One of the CPU core tries to execute this code and so acquires the

lock.

quoted

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.

quoted

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.

quoted

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.

quoted

I guess?timeslice expire case is not as same as preemption. Or may be

I am

quoted

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