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

From: "Nicholas Piggin" <[email-masked]>
Date: Tue, 09 May 2023 12:26:49 +1000

On Mon May 8, 2023 at 12:01 PM AEST, Rohan McLure wrote:
> 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 <[email-masked]>
> Signed-off-by: Rohan McLure <[email-masked]>
> ---
>  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);

Sigh, another atomic. It's in a slow path though so I won't get too
upset. Would be nice to add a comment here and revert it when KCSCAN
can be taught about this pattern though, so we don't lose it.

>  }
>  
>  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);

Where is the thread_idle_state concurrency coming from?

Thanks,
Nick