Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_" causes kernel crash

[peterz@infradead.org (Peter Zijlstra)]

On Thu, Aug 30, 2018@11:53:17AM +0000, Eugeniy Paltsev wrote:
> I can see crashes with LLSC enabled in both SMP running on 4 cores
> and SMP running on 1 core.

So you're running on LL/SC enabled hardware; that would make Will's
patch irrelevant (although still a good idea for the hardware that does
care about that spinlocked atomic crud).

Does something like the below cure things? That would confirm the
suggestion that the change to __clear_bit_unlock() is the curprit.

If that doesn't cure things, then we've been looking in entirely the
wrong place.

---
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index 3ae021368f48..79c6978152f8 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -57,12 +57,7 @@ static inline void clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
 static inline void __clear_bit_unlock(unsigned int nr,
 				      volatile unsigned long *p)
 {
-	unsigned long old;
-
-	p += BIT_WORD(nr);
-	old = READ_ONCE(*p);
-	old &= ~BIT_MASK(nr);
-	atomic_long_set_release((atomic_long_t *)p, old);
+	clear_bit_unlock(nr, p);
 }
 
 /**