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

[Paul E. McKenney]

On Mon, Oct 14, 2024 at 02:42:33PM +0530, Neeraj Upadhyay wrote:
> 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@xxxxxxxxxx>
> > Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> > Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Cc: <bpf@xxxxxxxxxxxxxxx>
> > ---
> >  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?

I would say that it is part of the upgrade.  The old logic worked if there
are only two flavors, but the cmpxchg() is required for more than two.

							Thanx, Paul

> - 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);
> >  }
>