Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor

[Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>]

On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
> Currently, there are only two flavors of readers, normal and NMI-safe.
> Very straightforward state updates suffice to check for erroneous
> mixing of reader flavors on a given srcu_struct structure.  This commit
> upgrades the checking in preparation for the addition of light-weight
> (as in memory-barrier-free) readers.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: <bpf@vger.kernel.org>
> ---
>  kernel/rcu/srcutree.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 18f2eae5e14bd..abe55777c4335 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>  		if (IS_ENABLED(CONFIG_PROVE_RCU))
>  			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
>  	}
> -	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
> +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
>  		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
>  	return sum;
>  }
> @@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
>  	sdp = raw_cpu_ptr(ssp->sda);
>  	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
>  	if (!old_reader_flavor_mask) {
> -		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
> -		return;
> +		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);

This looks to be separate independent fix?


- Neeraj

> +		if (!old_reader_flavor_mask)
> +			return;
>  	}
>  	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
>  }