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

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Date:   Thu, 26 Jan 2023 09:35:07 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        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 Thu, Jan 26, 2023 at 12:30:14PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/25/2023 11:52 PM, Alan Stern wrote:
> > 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
> > > [...]
> > > >    // 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.
> I don't think they're necessarily implemented in a compatible way, so
> 
> r = srcu_lock(s);
> srcu_up(s,r);
> 
> might not actually work, but would currently be ok'ed by LKMM.

In kernels built with CONFIG_PROVE_LOCKING=y (AKA built with lockdep
enabled), lockdep would complain about having an srcu_read_lock() with
no matching srcu_read_unlock().  Kernels built without lockdep (that is,
kernels actually used in production) would be happy with this.

So as Jonas suspects, this should be classified as not actually working.

>                                                                With
> different classes you could state
>   flag ~empty [Srcu-lock];srcu-rscs;[Srcu-up] as srcu-mismatch-lock-to-up
>   flag ~empty [Srcu-down];srcu-rscs;[Srcu-unlock] as
> srcu-mismatch-down-to-unlock
> 
> I think with the current implementation this code might work, but I don't
> feel like this is inherently true.
> 
> You could then also go ahead and define the "same CPU" requirement as a flag
> for lock and unlock specifically, like
>   flag ~empty [Srcu-lock];srcu-rscs & ext as srcu-lock-unlock-mismatch-CPU
> or so.
> 
> Best wishes, jonas
>