Fix CPU spinlock lockups on secondary CPU bringup

Secondary CPU bringup typically calls calibrate_delay() during its
initialization.  However, calibrate_delay() modifies a global variable
(loops_per_jiffy) used for udelay() and __delay().

A side effect of 71c696b1 ("calibrate: extract fall-back calculation
into own helper") introduced in the 2.6.39 merge window means that we
end up with a substantial period where loops_per_jiffy is zero.  This
causes the spinlock debugging code to malfunction:

	u64 loops = loops_per_jiffy * HZ;
	for (;;) {
		for (i = 0; i < loops; i++) {
			if (arch_spin_trylock(&lock->raw_lock))
				return;
			__delay(1);
		}
		...
	}

by never calling arch_spin_trylock() - resulting in the CPU locking
up in an infinite loop inside __spin_lock_debug().

Work around this by only writing to loops_per_jiffy only once we have
completed all the calibration decisions.

Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: <stable@kernel.org> (2.6.39-stable)
--
Better solutions (such as omitting the calibration for secondary CPUs,
or arranging for calibrate_delay() to return the LPJ value and leave
it to the caller to decide where to store it) are a possibility, but
would be much more invasive into each architecture.

I think this is the best solution for -rc and stable, but it should be
revisited for the next merge window.

 init/calibrate.c |   14 ++++++++------
 1 files changed, 8 insertions(+), 6 deletions(-)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 6 deletions

diff --git a/init/calibrate.c b/init/calibrate.c
index 2568d22a304e..aae2f40fea4c 100644
--- a/init/calibrate.c
+++ b/init/calibrate.c
@@ -245,30 +245,32 @@ recalibrate:
 
 void __cpuinit calibrate_delay(void)
 {
+        unsigned long lpj;
         static bool printed;
 
         if (preset_lpj) {
-                loops_per_jiffy = preset_lpj;
+                lpj = preset_lpj;
                 if (!printed)
                         pr_info("Calibrating delay loop (skipped) "
                                 "preset value.. ");
         } else if ((!printed) && lpj_fine) {
-                loops_per_jiffy = lpj_fine;
+                lpj = lpj_fine;
                 pr_info("Calibrating delay loop (skipped), "
                         "value calculated using timer frequency.. ");
-        } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
+        } else if ((lpj = calibrate_delay_direct()) != 0) {
                 if (!printed)
                         pr_info("Calibrating delay using timer "
                                 "specific routine.. ");
         } else {
                 if (!printed)
                         pr_info("Calibrating delay loop... ");
-                loops_per_jiffy = calibrate_delay_converge();
+                lpj = calibrate_delay_converge();
         }
         if (!printed)
                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
-                        loops_per_jiffy/(500000/HZ),
-                        (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);
+                        lpj/(500000/HZ),
+                        (lpj/(5000/HZ)) % 100, lpj);
 
+        loops_per_jiffy = lpj;
         printed = true;
 }