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SubjectRe: [RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.
On Mon, Jun 20, 2011 at 12:40:19PM +0100, Russell King - ARM Linux wrote:
> Ok. So loops_per_jiffy must be too small. My guess is you're using an
> older kernel without 71c696b1 (calibrate: extract fall-back calculation
> into own helper).

Right, this commit above helps show the problem - and it's fairly subtle.

It's a race condition. Let's first look at the spinlock debugging code.
It does this:

static void __spin_lock_debug(raw_spinlock_t *lock)
{
u64 i;
u64 loops = loops_per_jiffy * HZ;
for (;;) {
for (i = 0; i < loops; i++) {
if (arch_spin_trylock(&lock->raw_lock))
return;
__delay(1);
}
/* print warning */
}
}
If loops_per_jiffy is zero, we never try to grab the spinlock, because
we never enter the inner for loop. We immediately print a warning,
and re-execute the outer loop for ever, resulting in the CPU locking up
in this condition.

In theory, we should never see a zero loops_per_jiffy value, because it
represents the number of loops __delay() needs to delay by one jiffy and
clearly zero makes no sense.

However, calibrate_delay() does this (which x86 and ARM call on secondary
CPU startup):

calibrate_delay()
{
...
if (preset_lpj) {
} else if ((!printed) && lpj_fine) {
} else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
} else {
/* approximation/convergence stuff */
}
}
Now, before 71c696b, this used to be:

} else {
loops_per_jiffy = (1<<12);
So the window between calibrate_delay_direct() returning and setting
loops_per_jiffy to zero, and the re-initialization of loops_per_jiffy
was relatively short (maybe even the compiler optimized away the zero
write.)

However, after 71c696b, this now does:

} else {
if (!printed)
pr_info("Calibrating delay loop... ");
+ loops_per_jiffy = calibrate_delay_converge();
So, as loops_per_jiffy is not local to this function, the compiler has
to write out that zero value, before calling calibrate_delay_converge(),
and loops_per_jiffy only becomes non-zero _after_ calibrate_delay_converge()
has returned. This opens the window and allows the spinlock debugging
code to explode.

This patch closes the window completely, by only writing to loops_per_jiffy
only when we have a real value for it.

This allows me to boot 3.0.0-rc3 on Versatile Express (4 CPU) whereas
without this it fails with spinlock lockup and rcu problems.

init/calibrate.c | 14 ++++++++------
1 files changed, 8 insertions(+), 6 deletions(-)
diff --git a/init/calibrate.c b/init/calibrate.c
index 2568d22..aae2f40 100644
--- a/init/calibrate.c
+++ b/init/calibrate.c
@@ -245,30 +245,32 @@ static unsigned long __cpuinit calibrate_delay_converge(void)

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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