Thanks Rik,
On Mon, Mar 31, 2008 at 12:16 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> On Sun, 30 Mar 2008 23:24:50 +0800
> "Peter Teoh" <htmldeveloper@xxxxxxxxx> wrote:
>
> > Is there any guidelines on when and where can we insert printk() to do
> > debugging?
>
> You can use it pretty much anywhere.
>
Ah....so I supposed it updates the memory directly, no I/O is involved
right? Another daemon will then asynchronously read it from the
memory to write to the system file (/var/log/message*) - correct?
(this operation can sleep.) So since printk() DOES NOT sleep (as
implied by u) we can pretty use it anywhere?
Thanks in advance for the answer :-).
>
> > asmlinkage int vprintk(const char *fmt, va_list args)
> > {
>
> ...
>
>
> > preempt_disable();
> > /* This stops the holder of console_sem just where we want him */
> > raw_local_irq_save(flags);
> >
> > So we can see that printk() MUST NOT BE USED when the kernel is
> > preempt-disabled, as at the end of printk() it will be enabled again.
> >
> > MISUSE scenario: using printk() INSIDE a spin_lock()-spin_unlock() pair.
>
> Multiple preempt_disable+preempt_enable sequences can nest, because
> they simply keep a counter and preemption will not be enabled until
> the counter reaches zero (the outer-most preempt_disable call).
>
Ok, I am trying to understand this:
spin_lock()-->_spin_lock():
void __lockfunc _spin_lock(spinlock_t *lock)
{
preempt_disable();
spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
LOCK_CONTENDED(lock, _raw_spin_trylock, _raw_spin_lock);
}
For preempt_disable() (linux/preempt.h):
#define preempt_disable() \
do { \
inc_preempt_count(); \
barrier(); \
} while (0)
but inc_preempt_count() effectively just increase the
current_thread_info()->preempt_count.
and barrier() just put a barrier to cancel any potential internal CPU
instructions reordering through optimization. So question is WHERE
IS the preemption disabling? Ie, "cli" in x86 context?
(but inside spin_acquire(), which called lock_acquire(), there is a
check for irq_disabled(), and therefore the irq disabling must happen
before this spin_acquire(), right?)
> --
> All rights reversed.
>
erh.....u mean every time we quote your statements, you will pay us
money? :-).
if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
return 0;
--
Regards,
Peter Teoh
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