Messages in this thread | | | Date | Thu, 19 Jan 2023 17:04:49 -0500 | From | Alan Stern <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Thu, Jan 19, 2023 at 01:53:04PM -0800, Paul E. McKenney wrote: > On Thu, Jan 19, 2023 at 02:51:53PM -0500, Alan Stern wrote: > > 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.
But I'll put it in a separate patch, since it's not related to the main purpose of this change.
> > > (* 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?
No, and it's not to avoid confusion. It merely indicates that there has to be at least one instance of data or rf here; we will never have a case where the lock and the unlock are the same event.
> > > (* 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?
Right.
> > > 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.
Okay, good. In theory we could check for read_lock and read_unlock on different CPUs, but I don't think it's worth the trouble.
Alan
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