Messages in this thread |  | | Date | Sat, 14 Jan 2023 12:40:39 -0500 | From | Alan Stern <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Fri, Jan 13, 2023 at 12:32:41PM -0800, Paul E. McKenney wrote: > > Making LKMM correctly model all of this has been on my todo list for an > > embarrassingly long time. > > But there is no time like the present... > > Here is what mainline has to recognize SRCU read-side critical sections: > > ------------------------------------------------------------------------ > > (* 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 > > (* Validate nesting *) > flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking > flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking > > (* 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 > > ------------------------------------------------------------------------ > > And here is what I just now tried: > > ------------------------------------------------------------------------ > > (* Compute matching pairs of Srcu-lock and Srcu-unlock *) > let srcu-rscs = ([Srcu-lock] ; rfi ; [Srcu-unlock]) & loc
This doesn't make sense. Herd treats srcu_read_lock() as a load operation (it takes a pointer as argument and returns a value) and srcu_read_unlock() as a store operation (it takes both a pointer and a value as arguments and returns nothing). So you can't connect them with an rfi link; stores don't "read-from" loads.
I suppose you might be able to connect them with a data dependency, though. But then how would you handle situations where two unlock calls both use the value returned from a single lock call? You'd have to check explicitly that srcu-rscs connected each lock with only one unlock.
Alan
> (* Validate nesting *) > flag empty srcu-rscs as no-srcu-readers > flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking > flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking > > (* 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 > > ------------------------------------------------------------------------ > > This gets me "Flag no-srcu-readers" when running this litmus test: > > ------------------------------------------------------------------------ > > C C-srcu-nest-1 > > (* > * Result: Never > *) > > {} > > P0(int *x, int *y, struct srcu_struct *s) > { > int r1; > int r2; > int r3; > > r3 = srcu_read_lock(s); > r1 = READ_ONCE(*x); > srcu_read_unlock(s, r3); > r3 = srcu_read_lock(s); > r2 = READ_ONCE(*y); > srcu_read_unlock(s, r3); > } > > P1(int *x, int *y, struct srcu_struct *s) > { > WRITE_ONCE(*y, 1); > synchronize_srcu(s); > WRITE_ONCE(*x, 1); > } > > locations [0:r1] > exists (0:r1=1 /\ 0:r2=0) > > ------------------------------------------------------------------------ > > So what did I mess up this time? ;-) > > Thanx, Paul
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