Messages in this thread | | | Date | Fri, 10 Jun 2016 01:25:03 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH v3] powerpc: spinlock: Fix spin_unlock_wait() |
| |
On Thu, Jun 09, 2016 at 10:23:28PM +1000, Michael Ellerman wrote: > On Wed, 2016-06-08 at 15:59 +0200, Peter Zijlstra wrote: > > On Wed, Jun 08, 2016 at 11:49:20PM +1000, Michael Ellerman wrote: > > > > > > Ok; what tree does this go in? I have this dependent series which I'd > > > > like to get sorted and merged somewhere. > > > > > > Ah sorry, I didn't realise. I was going to put it in my next (which doesn't > > > exist yet but hopefully will early next week). > > > > > > I'll make a topic branch with just that commit based on rc2 or rc3? > > > > Works for me; thanks! > > Unfortunately the patch isn't 100%. > > It's causing some of my machines to lock up hard, which isn't surprising when > you look at the generated code for the non-atomic spin loop: > > c00000000009af48: 7c 21 0b 78 mr r1,r1 # HMT_LOW > c00000000009af4c: 40 9e ff fc bne cr7,c00000000009af48 <.do_exit+0x6d8> >
There is even no code checking for SHARED_PROCESSOR here, so I assume your config is !PPC_SPLPAR.
> Which is a spin loop waiting for a result in cr7, but with no comparison. > > The problem seems to be that we did: > > @@ -184,7 +184,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock) > if (arch_spin_value_unlocked(lock_val)) > goto out; > > - while (lock->slock) { > + while (!arch_spin_value_unlocked(*lock)) { > HMT_low(); > if (SHARED_PROCESSOR) > __spin_yield(lock); >
And as I also did an consolidation in this patch, we now share the same piece of arch_spin_unlock_wait(), so if !PPC_SPLPAR, the previous loop became:
while (!arch_spin_value_unlocked(*lock)) { HMT_low(); }
and given HMT_low() is not a compiler barrier. So the compiler may optimize out the loop..
> Which seems to be hiding the fact that lock->slock is volatile from the > compiler, even though arch_spin_value_unlocked() is inline. Not sure if that's > our bug or gcc's. >
I think arch_spin_value_unlocked() is not volatile because arch_spin_value_unlocked() takes the value of the lock rather than the address of the lock as its parameter, which makes it a pure function.
To fix this we can add READ_ONCE() for the read of lock value like the following:
while(!arch_spin_value_unlock(READ_ONCE(*lock))) { HMT_low(); ...
Or you prefer to simply using lock->slock which is a volatile variable already?
Or maybe we can refactor the code a little like this:
static inline void arch_spin_unlock_wait(arch_spinlock_t *lock) { arch_spinlock_t lock_val;
smp_mb();
/* * Atomically load and store back the lock value (unchanged). This * ensures that our observation of the lock value is ordered with * respect to other lock operations. */ __asm__ __volatile__( "1: " PPC_LWARX(%0, 0, %2, 0) "\n" " stwcx. %0, 0, %2\n" " bne- 1b\n" : "=&r" (lock_val), "+m" (*lock) : "r" (lock) : "cr0", "xer");
while (!arch_spin_value_unlocked(lock_val)) { HMT_low(); if (SHARED_PROCESSOR) __spin_yield(lock);
lock_val = READ_ONCE(*lock); } HMT_medium();
smp_mb(); }
> Will sleep on it. >
Bed time for me too, I will run more tests on the three proposals above tomorrow and see how things are going.
Regards, Boqun
> cheers > [unhandled content-type:application/pgp-signature] | |