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

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Message-ID: <abf6c382-7b70-4cdb-9227-7dfd21e60c45@amd.com>
Date: Mon, 14 Oct 2024 14:42:33 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: "Paul E. McKenney" <paulmck@...nel.org>, frederic@...nel.org,
 rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com, rostedt@...dmis.org,
 Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Kent Overstreet <kent.overstreet@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional
 reader flavor

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@...nel.org>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Kent Overstreet <kent.overstreet@...ux.dev>
> Cc: <bpf@...r.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);
>  }