linux-kernel - Re: [Patch 2/2] tools/memory-model: Provide exact SRCU semantics

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Date:   Wed, 25 Jan 2023 17:52:42 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [Patch 2/2] tools/memory-model: Provide exact SRCU semantics

On Wed, Jan 25, 2023 at 10:04:29PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/25/2023 9:21 PM, Alan Stern wrote:
> >   (* Validate nesting *)
> >   flag ~empty Srcu-lock \ domain(srcu-rscs) as unmatched-srcu-lock
> >   flag ~empty Srcu-unlock \ range(srcu-rscs) as unmatched-srcu-unlock
> > +flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-srcu-matches
> 
> Have you considered adding
> flag ~empty (srcu-rscs ; srcu-rscs^-1) \ id as mixed-srcu-cookie

I had not considered it.  You'd have to do something pretty bizarre if 
you wanted to trigger this warning, though.  Like:

	r1 = srcu_read_lock(s);
	r2 = srcu_read_lock(s);
	srcu_read_unlock(s, r1 + r2);

> Although I think one has to be intentionally trying to trick herd
> to be violating this. If herd could produce different cookies, this would be
> easy to detect just by the different-values flag you already have.

Unless you did: srcu_read_unlock(s, r1 + r2 * 0).  :-)

> >   (* Check for use of synchronize_srcu() inside an RCU critical section *)
> >   flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
> > @@ -80,11 +73,11 @@ flag ~empty different-values(srcu-rscs)
> >   (* Compute marked and plain memory accesses *)
> >   let Marked = (~M) | IW | Once | Release | Acquire | domain(rmw) | range(rmw) |
> > -		LKR | LKW | UL | LF | RL | RU
> > +		LKR | LKW | UL | LF | RL | RU | Srcu-lock | Srcu-unlock
> 
> Good catch! But why wasn't this necessary before? Is it only necessary now
> because the accesses became loads and stores (maybe to avoid data races?)

Exactly.  Before this those events weren't memory accesses at all.

> >   // SRCU
> > -srcu_read_lock(X)  __srcu{srcu-lock}(X)
> > -srcu_read_unlock(X,Y) { __srcu{srcu-unlock}(X,Y); }
> > +srcu_read_lock(X) __load{srcu-lock}(*X)
> > +srcu_read_unlock(X,Y) { __store{srcu-unlock}(*X,Y); }
> > +srcu_down_read(X) __load{srcu-lock}(*X)
> > +srcu_up_read(X,Y) { __store{srcu-unlock}(*X,Y); }
> 
> How do you feel about introducing Srcu-up and Srcu-down with this patch?

Why invent new classes for them?  They are literally the same operation 
as Srcu-lock and Srcu-unlock; the only difference is how the kernel's 
lockdep checker treats them.

> > +(* There should be no ordinary R or W accesses to spinlocks or SRCU structs *)
> > +let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock | Sync-srcu
> > +flag ~empty [M \ IW \ ALL-LOCKS] ; loc ; [ALL-LOCKS] as mixed-lock-accesses
> 
> Since this was pointed out by Boqun, would it be appropriate to mention him
> in the patch somehow?

True.  After we settle everything else, I'll add something to that 
effect.

Alan