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Subject[096/107] Fix CPU spinlock lockups on secondary CPU bringup
2.6.39-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Russell King <rmk+kernel@arm.linux.org.uk>

commit 1b19ca9f0bdab7d5035821e1ec8f39df9a6e3ee0 upstream.

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>
--
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.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
init/calibrate.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
--- a/init/calibrate.c
+++ b/init/calibrate.c
@@ -185,30 +185,32 @@ 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;
}



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