Messages in this thread | | | Date | Thu, 19 Jan 2023 15:03:20 -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 05:04:49PM -0500, Alan Stern wrote: > 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.
Even better!
> > > (* 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.
Got it, thank you!
> > > (* 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.
Given that lockdep already complains about that sort of thing in the Linux kernel, agreed, it is not worth much trouble at all.
Thanx, Paul
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