On 15/09/2010 22:15, "Roger Cruz" <roger.cruz@xxxxxxxxxxxxxxxxxxx> wrote:
> Hi Keir,
>
> Good to know that..the NMI handler (nmi_watchdog_tick) does have a printk but
> I supposed it is ok since we are going to reboot after, so we are not
> returning to the other code.
Exactly. And note that we bust the console_lock, via console_force_unlock(),
first.
-- Keir
> I assume that putting a printk in a timer
> function callback (set up with init_timer & set_timer, etc) or any other
> interrupt handler would not be safe either then.
>
> Roger
>
>
>
> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Wed 9/15/2010 5:03 PM
> To: Roger Cruz; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Question about printk implementation.
>
> On 15/09/2010 21:24, "Roger Cruz" <roger.cruz@xxxxxxxxxxxxxxxxxxx> wrote:
>
>> I was looking over the implementation of printk (xenunstable) and I have a
>> question about the spin_lock_recursive use and the static "buf" variable in
>> this routine. Suppose that that in a single processor system the hypervisor
>> is printing using this printk routine and has acquired the console_lock. Is
>> it possible for a high level interrupt, like an NMI (just as an example, any
>> other interrupts that preempt the current work in the hypervisor will do), to
>> come in and then use printk. Because we are in single CPU mode, the
>> spin_lock_recursive will succeed (if I interpreted the code correctly). When
>> the higher level interrupt exits and returns to the hypervisor code that was
>> previously printing, the contents of the static "buf" would have changed. Is
>> this a possibility??
>
> Well yeah, you know what? Don't do that then! Our printk (like very many Xen
> functions) is not NMI safe.
>
> -- Keir
>
>> Thanks
>> Roger R. Cruz
>>
>> void printk(const char *fmt, ...)
>> {
>> static char buf[1024]; <<<<<---------------
>> static int start_of_line = 1, do_print;
>>
>> va_list args;
>> char *p, *q;
>> unsigned long flags;
>>
>> /* console_lock can be acquired recursively from __printk_ratelimit(). */
>> local_irq_save(flags); <<<<-----------------
>> spin_lock_recursive(&console_lock);
>>
>> va_start(args, fmt);
>> (void)vsnprintf(buf, sizeof(buf), fmt, args);
>> va_end(args);
>>
>> p = buf;
>>
>> while ( (q = strchr(p, '\n')) != NULL )
>> {
>> *q = '\0';
>> if ( start_of_line )
>> do_print = printk_prefix_check(p, &p);
>> if ( do_print )
>> {
>> if ( start_of_line )
>> printk_start_of_line();
>> __putstr(p);
>> __putstr("\n");
>> }
>> start_of_line = 1;
>> p = q + 1;
>>
>
>
>
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