Messages in this thread | | | Date | Thu, 19 Jan 2023 13:53:04 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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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
Fair point, and for example we already recommend emulating call_rcu() using similar release-acquire tricks.
> 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! ;-)
> 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
This renaming makes sense to me.
> (* 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
The point of the "+" instead of the "*" is to avoid LKMM being confused by an srcu_read_lock() immediately preceding an unrelated srcu_read_unlock(), right? Or am I missing something more subtle?
> (* 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)*
The "[~ Srcu-unlock]" matches the store that bridges the data and rfi", correct?
> 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); }
And here srcu_down_read() and srcu_up_read() are synonyms for srcu_read_lock() and srcu_read_unlock(), respectively, which I believe should suffice.
> synchronize_srcu(X) { __srcu{sync-srcu}(X); } > synchronize_srcu_expedited(X) { __srcu{sync-srcu}(X); }
So this looks quite reasonable to me.
Thanx, Paul
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