Re: newbie questions about while (1) in kernel mode and spinlocks

Robert Hancock
2006-12-21T14:31:58Z

Sorin Manolache wrote:
>
> The Linux Device Drivers book says that a spin_lock should not be
> shared between a process and an interrupt handler. The explanation is
> that the process may hold the lock, an interrupt occurs, the interrupt
> handler spins on the lock held by the process and the system freezes.
> Why should it freeze? Isn't it possible for the interrupt handler to
> re-enable interrupts as its first thing, then to spin at the lock, the
> timer interrupt to preempt the interrupt handler and to relinquish
> control to the process which in turn will finish its critical section
> and release the lock, making way for the interrupt handler to
> continue.

When the timer interrupt finishes, it's not going to return control to
the process, it's going to return control to what it interrupted which
is the interrupt handler in your code which will just continue spinning.
Interrupt handlers are not preemptible by anything other than other
interrupt handlers (and then only in some cases) so it will sit spinning
on that lock forever.

--
Robert Hancock Saskatoon, SK, Canada
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