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Date:   Fri, 20 Jan 2023 10:20:11 +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>
References: <Y8gjUKoHxqR9+7Hx@rowland.harvard.edu>
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 <Y8mfWTX7V69pAwo8@rowland.harvard.edu>
 <20230120035521.GA319650@paulmck-ThinkPad-P17-Gen-1>
From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
In-Reply-To: <20230120035521.GA319650@paulmck-ThinkPad-P17-Gen-1>
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On 1/20/2023 4:55 AM, 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.
>>
>> With the patch below, the results were as expected:
>>
>> Test C-srcu-nest-6 Allowed
>> States 3
>> 0:r1=0; 0:r2=0; 1:r1=0;
>> 0:r1=0; 0:r2=1; 1:r1=0;
>> 0:r1=0; 0:r2=1; 1:r1=1;
>> No
>> Witnesses
>> Positive: 0 Negative: 3
>> Condition exists (1:r1=1 /\ 0:r2=0)
>> Observation C-srcu-nest-6 Never 0 3
>> Time C-srcu-nest-6 0.04
>> Hash=2b010cf3446879fb84752a6016ff88c5
>>
>> 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.
>>
>> Alan
>>
>>
>>
>> Index: usb-devel/tools/memory-model/linux-kernel.bell
>> ===================================================================
>> --- usb-devel.orig/tools/memory-model/linux-kernel.bell
>> +++ usb-devel/tools/memory-model/linux-kernel.bell
>> @@ -53,38 +53,30 @@ let rcu-rscs = let rec
>>   	in matched
>>   
>>   (* Validate nesting *)
>> -flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-locking
>> -flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-locking
>> +flag ~empty Rcu-lock \ domain(rcu-rscs) as unbalanced-rcu-lock
>> +flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-unlock
>>   
>>   (* Compute matching pairs of nested Srcu-lock and Srcu-unlock *)
>> -let srcu-rscs = let rec
>> -	    unmatched-locks = Srcu-lock \ domain(matched)
>> -	and unmatched-unlocks = Srcu-unlock \ range(matched)
>> -	and unmatched = unmatched-locks | unmatched-unlocks
>> -	and unmatched-po = ([unmatched] ; po ; [unmatched]) & loc
>> -	and unmatched-locks-to-unlocks =
>> -		([unmatched-locks] ; po ; [unmatched-unlocks]) & loc
>> -	and matched = matched | (unmatched-locks-to-unlocks \
>> -		(unmatched-po ; unmatched-po))
>> -	in matched
>> +let srcu-rscs = ([Srcu-lock] ; (data | rf)+ ; [Srcu-unlock]) & loc
>>   
>>   (* Validate nesting *)
>> -flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
>> -flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking
>> +flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-lock
>> +flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-unlock
>> +flag ~empty (srcu-rscs^-1 ; srcu-rscs) \ id as multiple-srcu-matches
>>   
>>   (* Check for use of synchronize_srcu() inside an RCU critical section *)
>>   flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
>>   
>>   (* Validate SRCU dynamic match *)
>> -flag ~empty different-values(srcu-rscs) as srcu-bad-nesting
>> +flag ~empty different-values(srcu-rscs) as bad-srcu-value-match
>>   
>>   (* 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
>>   let Plain = M \ Marked
>>   
>>   (* Redefine dependencies to include those carried through plain accesses *)
>> -let carry-dep = (data ; rfi)*
>> +let carry-dep = (data ; [~ Srcu-unlock] ; rfi)*
>>   let addr = carry-dep ; addr
>>   let ctrl = carry-dep ; ctrl
>>   let data = carry-dep ; data
>> Index: usb-devel/tools/memory-model/linux-kernel.def
>> ===================================================================
>> --- usb-devel.orig/tools/memory-model/linux-kernel.def
>> +++ usb-devel/tools/memory-model/linux-kernel.def
>> @@ -49,8 +49,10 @@ synchronize_rcu() { __fence{sync-rcu}; }
>>   synchronize_rcu_expedited() { __fence{sync-rcu}; }
>>   
>>   // 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); }
>>   synchronize_srcu(X)  { __srcu{sync-srcu}(X); }
>>   synchronize_srcu_expedited(X)  { __srcu{sync-srcu}(X); }
> And for some initial tests:
>
> https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-1.litmus
>
> 	"Flag multiple-srcu-matches" but otherwise OK.
> 	As a "hail Mary" exercise, I used r4 for the second SRCU
> 	read-side critical section, but this had no effect.
> 	(This flag is expected and seen for #4 below.)

This is because srcu_lock/srcu_unlock are reads and writes, and so you 
get the accidental rf relation here I was talking about earlier.
In particular your first lock() is linked by  data ; rf ; data to the 
second unlock(), which therefore seems to have data coming in from two 
sources.

You would be better off moving the carry-dep/data definitions higher in 
the file,

-let carry-dep = (data ; rfi)*
+let carry-dep = (data ; [~ Srcu-unlock] ; rfi)*
  let addr = carry-dep ; addr
  let ctrl = carry-dep ; ctrl
  let data = carry-dep ; data

and then defining

+let srcu-rscs = ([Srcu-lock] ; data ; [Srcu-unlock]) & loc

Note here I'm just using the freshly redefined data, instead of the (data;rf)+


best wishes, jonas