As I remember timer interrupt as well is an NMI so, it is
possible (although not advised) to call schedule function while
holding spinlock on same core.

spin_lock_irqsave();
schedule();
spin_lock_irqrestore();

sorry for confusion created by my last mail, looks like its only watchdog
timer which are NMI and above function call to schedule() does not require
any help from scheduler's timer interrupts (not sure if same timer hardware
is used for watchdog and scheduler timer interrupts, my guess is both may
share the hardware, but different IRQL line, one maskable other
non-maskable). However it is still possible to voluntarily schedule the CPU
core by currently executing code even with spin_locks disabled.

Rajat

On Fri, Jan 7, 2011 at 3:01 PM, nilesh [off-list ref] wrote:

On Fri, 2011-01-07 at 13:33 +0530, anish singh wrote:

quoted

On Fri, Jan 7, 2011 at 1:19 PM, nilesh [off-list ref]
wrote:
        On Fri, 2011-01-07 at 13:05 +0530, Rajat Sharma wrote:
        > As I remember timer interrupt as well is an NMI so, it is
        possible
        > (although not advised) to call schedule function while
        holding
        > spinlock on same core.
        >
        > spin_lock_irqsave();
        > schedule();
        > spin_lock_irqrestore();
        >
        > however if you have debugging options turned on like
        > CONFIG_DEBUG_SPINLOCK, you may likely get kernel warning for
        > 'scheduling in atomic context'.
        >
        > Then what can happen if this core is allowed to switched to
        new
        > process? Consider the case where new process as well tries
        to aquire
        > same spin_lock() which new process can not aquire and start
        spinning
        > for the lock for ever :). Likewise, other cores will also
        get locked
        > down.
        >
        > However stil you can detect softlockup through NMI watchdog.


        >>Sorry if I am building up the confusion here. But as Dave
        Hylands
        >>initially mentioned, there will be no timer interrupt. So
        shouldn't the
        >>NMI watchdog get triggered then? No interrupts -> system
        freeze -> NMI
        >>Wdt reboot.

In my opinion(uninformed ) NMI watchdog will be triggered only in case
where you are holding a spinlock.It will not be triggered just because
timer interrupts are disabled due to holding a spinlock.

No, what I meant is - we have masked all the interrupts before holding
the spinlock (and not even getting the timer interrupts) so it's as good
as system freeze. And we should trigger the NMI watchdog, isn't it?

--
Thanks,
Nilesh

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20110107/112a60ea/attachment-0001.html