On Sat, 2002-04-20 at 06:27, Paul Mackerras wrote:
> Thus if we have CONFIG_SMP and CONFIG_PREEMPT, there is a small but
> non-zero probability that in_interrupt() will give the wrong answer if
> it is called with preemption enabled. If the process gets scheduled
> from cpu A to cpu B between calling smp_processor_id() and evaluating
> local_irq_count(cpu) or local_bh_count(), and cpu A then happens to be
> in interrupt context at the point where the process resumes on cpu B,
> then in_interrupt() will incorrectly return 1.
Looks like you are probably right ...
> One idea I had is to use a couple of bits in
> current_thread_info()->flags to indicate whether local_irq_count and
> local_bh_count are non-zero for the current cpu. These bits could be
> tested safely without having to disable preemption.
For now we can just do this,
--- linux-2.5.8/include/asm-i386/hardirq.h Sun Apr 14 15:18:55 2002
+++ linux/include/asm-i386/hardirq.h Mon Apr 22 14:56:29 2002
@@ -21,8 +21,10 @@
* Are we in an interrupt context? Either doing bottom half
* or hardware interrupt processing?
*/
-#define in_interrupt() ({ int __cpu = smp_processor_id(); \
- (local_irq_count(__cpu) + local_bh_count(__cpu) != 0); })
+#define in_interrupt() ({ int __cpu; preempt_disable(); \
+ __cpu = smp_processor_id(); \
+ (local_irq_count(__cpu) + local_bh_count(__cpu) != 0); \
+ preempt_enable(); })
#define in_irq() (local_irq_count(smp_processor_id()) != 0)
Or perhaps leave the code as-is but make the rule preemption needs to be
disabled before calling (either implicitly or explicitly). I.e., via a
call to preempt_disable or because interrupts are disabled, a lock is
held, etc ...
> In fact almost all uses of local_irq_count() and local_bh_count() are
> for the current cpu; the exceptions are the irqs_running() function
> and some debug printks. Maybe the irq and bh counters themselves
> could be put into the thread_info struct, if irqs_running could be
> implemented another way.
One thing Linus, DaveM, and I discussed a while back was actually
getting rid of the irq and bh counts completely and folding them into
preempt_count. I am interested in this...
Robert Love
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