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Date:   Fri, 20 Jan 2023 10:43:10 +0100
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Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
To:     paulmck@kernel.org, Alan Stern <stern@rowland.harvard.edu>
Cc:     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>
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From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
In-Reply-To: <20230119215304.GA2948950@paulmck-ThinkPad-P17-Gen-1>
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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 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?
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.

Best wishes,
   jonas