On 6/20/2011 7:53 PM, Russell King - ARM Linux wrote:
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(-)
I am away from my board now. Will test this change.
btw, the online-active race is still open even with this patch close
and should be fixed.
Regards
Santosh
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