rtc: fix deadlock

if get_rtc_time() is _ever_ called with IRQs off, we deadlock badly in it, waiting for jiffies to increment. So make the code more robust by doing an explicit mdelay(20). This solves a very hard to reproduce/debug hard lockup reported by Mikael Pettersson. Reported-by: Mikael Pettersson <mikpe@it.uu.se> Signed-off-by: Ingo Molnar <mingo@elte.hu>

rtc: fix deadlock
if get_rtc_time() is _ever_ called with IRQs off, we deadlock badly in it, waiting for jiffies to increment. So make the code more robust by doing an explicit mdelay(20). This solves a very hard to reproduce/debug hard lockup reported by Mikael Pettersson. Reported-by: Mikael Pettersson <mikpe@it.uu.se> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Ingo Molnar
1 parent 3c4fbe5e01
Showing 1 changed file with 4 additions and 8 deletions Side-by-side Diff
include/asm-generic/rtc.h
@@ -15,6 +15,7 @@
 #include <linux/mc146818rtc.h>
 #include <linux/rtc.h>
 #include <linux/bcd.h>
+#include <linux/delay.h>
  
 #define RTC_PIE 0x40		/* periodic interrupt enable */
 #define RTC_AIE 0x20		/* alarm interrupt enable */
@@ -43,7 +44,6 @@
  
 static inline unsigned int get_rtc_time(struct rtc_time *time)
 {
-	unsigned long uip_watchdog = jiffies;
 	unsigned char ctrl;
 	unsigned long flags;
  
  
@@ -53,19 +53,15 @@
  
 	/*
 	 * read RTC once any update in progress is done. The update
-	 * can take just over 2ms. We wait 10 to 20ms. There is no need to
+	 * can take just over 2ms. We wait 20ms. There is no need to
 	 * to poll-wait (up to 1s - eeccch) for the falling edge of RTC_UIP.
 	 * If you need to know *exactly* when a second has started, enable
 	 * periodic update complete interrupts, (via ioctl) and then 
 	 * immediately read /dev/rtc which will block until you get the IRQ.
 	 * Once the read clears, read the RTC time (again via ioctl). Easy.
 	 */
-
-	if (rtc_is_updating() != 0)
-		while (jiffies - uip_watchdog < 2*HZ/100) {
-			barrier();
-			cpu_relax();
-		}
+	if (rtc_is_updating())
+		mdelay(20);
  
 	/*
 	 * Only the values that we read from the RTC are set. We leave
...	...	@@ -15,6 +15,7 @@
15	15	#include <linux/mc146818rtc.h>
16	16	#include <linux/rtc.h>
17	17	#include <linux/bcd.h>
	18	+#include <linux/delay.h>
18	19
19	20	#define RTC_PIE 0x40 /* periodic interrupt enable */
20	21	#define RTC_AIE 0x20 /* alarm interrupt enable */
...	...	@@ -43,7 +44,6 @@
43	44
44	45	static inline unsigned int get_rtc_time(struct rtc_time *time)
45	46	{
46		- unsigned long uip_watchdog = jiffies;
47	47	unsigned char ctrl;
48	48	unsigned long flags;
49	49
50	50
...	...	@@ -53,19 +53,15 @@
53	53
54	54	/*
55	55	* read RTC once any update in progress is done. The update
56		- * can take just over 2ms. We wait 10 to 20ms. There is no need to
	56	+ * can take just over 2ms. We wait 20ms. There is no need to
57	57	* to poll-wait (up to 1s - eeccch) for the falling edge of RTC_UIP.
58	58	* If you need to know exactly when a second has started, enable
59	59	* periodic update complete interrupts, (via ioctl) and then
60	60	* immediately read /dev/rtc which will block until you get the IRQ.
61	61	* Once the read clears, read the RTC time (again via ioctl). Easy.
62	62	*/
63		-
64		- if (rtc_is_updating() != 0)
65		- while (jiffies - uip_watchdog < 2*HZ/100) {
66		- barrier();
67		- cpu_relax();
68		- }
	63	+ if (rtc_is_updating())
	64	+ mdelay(20);
69	65
70	66	/*
71	67	* Only the values that we read from the RTC are set. We leave