[PATCH]new_TSC_based_delay_tsc()

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hi all,

Please ignore the previous patch with the same subject.
It has a bug that can manifest itself in the very exotic case
when each do {} while() iteration executes on different cpu
leading to potentially infinite loop.

This is a patch based on the Ingo's idea/patch to track 
delay_tsc() migration to another cpu by comparing 
smp_processor_id(). It is against kernel-2.6.24-rc3.

What is different:
1. Using unsigned (instead of long) to unify for i386/x86_64.
2. Minimal preempt_disable/enable() critical sections
   (more room for preemption)
3. some statements have been rearranged, to account for
    possible under/overflow of left/TSC

Tested on both: 32/64 bit SMP PREEMPT kernel-2.6.24-rc3

Comments, please.

Signed-off-by: Marin Mitov <[email protected]>
=========================================
--- a/arch/x86/lib/delay_32.c	2007-11-18 08:14:05.000000000 +0200
+++ b/arch/x86/lib/delay_32.c	2007-11-20 19:03:52.000000000 +0200
@@ -38,18 +38,42 @@
 		:"0" (loops));
 }
 
-/* TSC based delay: */
+/* TSC based delay:
+ *
+ * We are careful about preemption as TSC's are per-CPU.
+ */
 static void delay_tsc(unsigned long loops)
 {
-	unsigned long bclock, now;
+	unsigned prev, prev_1, now;
+	unsigned left = loops;
+	unsigned prev_cpu, cpu;
+
+	preempt_disable();
+	rdtscl(prev);
+	prev_cpu = smp_processor_id();
+	preempt_enable();
+	now = prev;
 
-	preempt_disable();		/* TSC's are per-cpu */
-	rdtscl(bclock);
 	do {
 		rep_nop();
+
+		left -= now - prev;
+		prev = now;
+
+		preempt_disable();
+		rdtscl(prev_1);
+		cpu = smp_processor_id();
 		rdtscl(now);
-	} while ((now-bclock) < loops);
-	preempt_enable();
+		preempt_enable();
+
+		if (prev_cpu != cpu){
+			/*
+			 * We have migrated, forget prev_cpu's tsc reading
+			 */
+			prev = prev_1;
+			prev_cpu = cpu;
+		}
+	} while ((now-prev) < left);
 }
 
 /*
--- a/arch/x86/lib/delay_64.c	2007-11-18 08:14:40.000000000 +0200
+++ b/arch/x86/lib/delay_64.c	2007-11-20 19:47:29.000000000 +0200
@@ -26,18 +26,42 @@
 	return 0;
 }
 
+/* TSC based delay:
+ *
+ * We are careful about preemption as TSC's are per-CPU.
+ */
 void __delay(unsigned long loops)
 {
-	unsigned bclock, now;
+	unsigned prev, prev_1, now;
+	unsigned left = loops;
+	unsigned prev_cpu, cpu;
+
+	preempt_disable();
+	rdtscl(prev);
+	prev_cpu = smp_processor_id();
+	preempt_enable();
+	now = prev;
 
-	preempt_disable();		/* TSC's are pre-cpu */
-	rdtscl(bclock);
 	do {
-		rep_nop(); 
+		rep_nop();
+
+		left -= now - prev;
+		prev = now;
+
+		preempt_disable();
+		rdtscl(prev_1);
+		cpu = smp_processor_id();
 		rdtscl(now);
-	}
-	while ((now-bclock) < loops);
-	preempt_enable();
+		preempt_enable();
+
+		if (prev_cpu != cpu){
+			/*
+			 * We have migrated, forget prev_cpu's tsc reading
+			 */
+			 prev = prev_1;
+			 prev_cpu = cpu;
+		}
+	} while ((now-prev) < left);
 }
 EXPORT_SYMBOL(__delay);
 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux