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Date:   Fri, 20 Jan 2023 07:39:09 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
Message-ID: <20230120153909.GF2948950@paulmck-ThinkPad-P17-Gen-1>
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On Fri, Jan 20, 2023 at 10:43:10AM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/19/2023 10:53 PM, Paul E. McKenney wrote:
> > On Thu, Jan 19, 2023 at 02:51:53PM -0500, Alan Stern wrote:
> > > On Thu, Jan 19, 2023 at 10:41:07AM -0800, Paul E. McKenney wrote:
> > > > In contrast, this actually needs srcu_down_read() and srcu_up_read():
> > > > 
> > > > ------------------------------------------------------------------------
> > > > 
> > > > C C-srcu-nest-6
> > > > 
> > > > (*
> > > >   * Result: Never
> > > >   *
> > > >   * Flag unbalanced-srcu-locking
> > > >   * This would be valid for srcu_down_read() and srcu_up_read().
> > > >   *)
> > > > 
> > > > {}
> > > > 
> > > > P0(int *x, int *y, struct srcu_struct *s1, int *idx)
> > > > {
> > > > 	int r2;
> > > > 	int r3;
> > > > 
> > > > 	r3 = srcu_down_read(s1);
> > > > 	WRITE_ONCE(*idx, r3);
> > > > 	r2 = READ_ONCE(*y);
> > > > }
> > > > 
> > > > P1(int *x, int *y, struct srcu_struct *s1, int *idx)
> > > > {
> > > > 	int r1;
> > > > 	int r3;
> > > > 
> > > > 	r1 = READ_ONCE(*x);
> > > > 	r3 = READ_ONCE(*idx);
> > > > 	srcu_up_read(s1, r3);
> > > > }
> > > > 
> > > > P2(int *x, int *y, struct srcu_struct *s1)
> > > > {
> > > > 	WRITE_ONCE(*y, 1);
> > > > 	synchronize_srcu(s1);
> > > > 	WRITE_ONCE(*x, 1);
> > > > }
> > > > 
> > > > locations [0:r1]
> > > > exists (1:r1=1 /\ 0:r2=0)
> > > I modified this litmus test by adding a flag variable with an
> > > smp_store_release in P0, an smp_load_acquire in P1, and a filter clause
> > > to ensure that P1 reads the flag and idx from P1.
> 
> This sounds like good style.
> I suppose this is already flagged as mismatched srcu_unlock(), in case you
> accidentally read from the initial write?

It might, except that a filter clause excludes this case.  Here is the
updated test:

C C-srcu-nest-6

(*
 * Result: Never
 *
 * This would be valid for srcu_down_read() and srcu_up_read().
 *)

{}

P0(int *x, int *y, struct srcu_struct *s1, int *idx, int *f)
{
	int r2;
	int r3;

	r3 = srcu_down_read(s1);
	WRITE_ONCE(*idx, r3);
	r2 = READ_ONCE(*y);
	smp_store_release(f, 1);
}

P1(int *x, int *y, struct srcu_struct *s1, int *idx, int *f)
{
	int r1;
	int r3;
	int r4;

	r4 = smp_load_acquire(f);
	r1 = READ_ONCE(*x);
	r3 = READ_ONCE(*idx);
	srcu_up_read(s1, r3);
}

P2(int *x, int *y, struct srcu_struct *s1)
{
	WRITE_ONCE(*y, 1);
	synchronize_srcu(s1);
	WRITE_ONCE(*x, 1);
}

locations [0:r1]
filter (1:r4=1)
exists (1:r1=1 /\ 0:r2=0)

> > > It turns out that the idea of removing rf edges from Srcu-unlock events
> > > doesn't work well.  The missing edges mess up herd's calculation of the
> > > fr relation and the coherence axiom.  So I've gone back to filtering
> > > those edges out of carry-dep.
> > > 
> > > Also, Boqun's suggestion for flagging ordinary accesses to SRCU
> > > structures no longer works, because the lock and unlock operations now
> > > _are_ normal accesses.  I removed that check too, but it shouldn't hurt
> > > much because I don't expect to encounter litmus tests that try to read
> > > or write srcu_structs directly.
> > Agreed.  I for one would definitely have something to say about an
> > SRCU-usage patch that directly manipulated a srcu_struct structure!  ;-)
> 
> Wouldn't the point of having it being flagged be that herd (or similar
> tools) would be having something to say long before it has to reach your
> pair of eyes?

That would of course be even better.

> I don't think Boqun's patch is hard to repair.
> Besides the issue you mention, I think it's also missing Sync-srcu, which
> seems to be linked by loc based on its first argument.
> 
> How about something like this?
> 
> let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock |
> Sync-srcu   flag ~empty ~[ALL_LOCKS | IW] ; loc ; [ALL-LOCKS] as
> mixed-lock-accesses
> 
> If you're using something that isn't a lock or intial write on the same location as a lock, you get the flag.

Wouldn't that unconditionally complain about the first srcu_read_lock()
in a given process?  Or am I misreading those statements?

							Thanx, Paul