[PATCH 07/12] powerpc: powernv: Fix KCSAN datarace warnings on idle_state contention

[Rohan McLure]

The idle_state entry in the PACA on PowerNV features a bit which is
atomically tested and set through ldarx/stdcx. to be used as a spinlock.
This lock then guards access to other bit fields of idle_state. KCSAN
cannot differentiate between any of these bitfield accesses as they all
are implemented by 8-byte store/load instructions, thus cores contending
on the bit-lock appear to data race with modifications to idle_state.

Separate the bit-lock entry from the data guarded by the lock to avoid
the possibility of data races being detected by KCSAN.

Suggested-by: Nicholas Piggin <npiggin at gmail.com>
Signed-off-by: Rohan McLure <rmclure at ibm.com>
---
 arch/powerpc/include/asm/paca.h       |  1 +
 arch/powerpc/platforms/powernv/idle.c | 20 +++++++++++---------
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index da0377f46597..cb325938766a 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -191,6 +191,7 @@ struct paca_struct {
 #ifdef CONFIG_PPC_POWERNV
 	/* PowerNV idle fields */
 	/* PNV_CORE_IDLE_* bits, all siblings work on thread 0 paca */
+	unsigned long idle_lock; /* A value of 1 means acquired */
 	unsigned long idle_state;
 	union {
 		/* P7/P8 specific fields */
diff --git a/arch/powerpc/platforms/powernv/idle.c b/arch/powerpc/platforms/powernv/idle.c
index 841cb7f31f4f..97dbb7bc2b00 100644
--- a/arch/powerpc/platforms/powernv/idle.c
+++ b/arch/powerpc/platforms/powernv/idle.c
@@ -246,9 +246,9 @@ static inline void atomic_lock_thread_idle(void)
 {
 	int cpu = raw_smp_processor_id();
 	int first = cpu_first_thread_sibling(cpu);
-	unsigned long *state = &paca_ptrs[first]->idle_state;
+	unsigned long *lock = &paca_ptrs[first]->idle_lock;
 
-	while (unlikely(test_and_set_bit_lock(NR_PNV_CORE_IDLE_LOCK_BIT, state)))
+	while (unlikely(test_and_set_bit_lock(NR_PNV_CORE_IDLE_LOCK_BIT, lock)))
 		barrier();
 }
 
@@ -258,29 +258,31 @@ static inline void atomic_unlock_and_stop_thread_idle(void)
 	int first = cpu_first_thread_sibling(cpu);
 	unsigned long thread = 1UL << cpu_thread_in_core(cpu);
 	unsigned long *state = &paca_ptrs[first]->idle_state;
+	unsigned long *lock = &paca_ptrs[first]->idle_lock;
 	u64 s = READ_ONCE(*state);
 	u64 new, tmp;
 
-	BUG_ON(!(s & PNV_CORE_IDLE_LOCK_BIT));
+	BUG_ON(!(READ_ONCE(*lock) & PNV_CORE_IDLE_LOCK_BIT));
 	BUG_ON(s & thread);
 
 again:
-	new = (s | thread) & ~PNV_CORE_IDLE_LOCK_BIT;
+	new = s | thread;
 	tmp = cmpxchg(state, s, new);
 	if (unlikely(tmp != s)) {
 		s = tmp;
 		goto again;
 	}
+	clear_bit_unlock(NR_PNV_CORE_IDLE_LOCK_BIT, lock);
 }
 
 static inline void atomic_unlock_thread_idle(void)
 {
 	int cpu = raw_smp_processor_id();
 	int first = cpu_first_thread_sibling(cpu);
-	unsigned long *state = &paca_ptrs[first]->idle_state;
+	unsigned long *lock = &paca_ptrs[first]->idle_lock;
 
-	BUG_ON(!test_bit(NR_PNV_CORE_IDLE_LOCK_BIT, state));
-	clear_bit_unlock(NR_PNV_CORE_IDLE_LOCK_BIT, state);
+	BUG_ON(!test_bit(NR_PNV_CORE_IDLE_LOCK_BIT, lock));
+	clear_bit_unlock(NR_PNV_CORE_IDLE_LOCK_BIT, lock);
 }
 
 /* P7 and P8 */
@@ -380,9 +382,9 @@ static unsigned long power7_idle_insn(unsigned long type)
 		sprs.uamor	= mfspr(SPRN_UAMOR);
 	}
 
-	local_paca->thread_idle_state = type;
+	WRITE_ONCE(local_paca->thread_idle_state, type);
 	srr1 = isa206_idle_insn_mayloss(type);		/* go idle */
-	local_paca->thread_idle_state = PNV_THREAD_RUNNING;
+	WRITE_ONCE(local_paca->thread_idle_state, PNV_THREAD_RUNNING);
 
 	WARN_ON_ONCE(!srr1);
 	WARN_ON_ONCE(mfmsr() & (MSR_IR|MSR_DR));
-- 
2.37.2