Messages in this thread Patch in this message | | | Date | Fri, 20 Jan 2023 07:32:42 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Fri, Jan 20, 2023 at 01:51:01PM +0100, Jonas Oberhauser wrote: > I'm not going to get it right today, am I?
Believe me, I know that feeling! Open-source development is therefore an extremely good character-building exercise. At least that is what I keep telling myself. ;-)
> +let srcu-rscs = ([Srcu-lock] ; (data ; [~ Srcu-unlock] ; rfe) * ; data ; > [Srcu-unlock]) & loc > > I see now that I copied the format from your message but without realizing > the original had a `|` where I have a `;`. > I hope this version is finally right and perhaps more natural than the (data > | rf) version, considering rf can't actually appear in most places and this > more closely matches carry-dep;data. > But of course feel free to use > +let srcu-rscs = ([Srcu-lock] ; (data | [~ Srcu-unlock] ; rf)+ ; > [Srcu-unlock]) & loc > instead if you prefer.
Ah, herd7 could see an rf link between any srcu_read_unlock() and any "later" srcu_read_lock(), couldn't it? Good catch!!!
I took this last one, adding parentheses that might well be unnecessary. (You see, herd7 was complaining about cut-and-paste, possibly due to alternative character sets, so I indulged in a bit of diagnostic-driven development.)
let srcu-rscs = ([Srcu-lock] ; (data | ([~ Srcu-unlock] ; rf))+ ; [Srcu-unlock]) & loc
The reason for favoring "rf" over "rfe" is the possibility of a litmus test where the process containing the srcu_down_read() sometimes but not always also has the matching srcu_up_read(). Perhaps a pair of "if" statements control which process does the matching srcu_up_read().
With this change, all of the C-srcu-nest-*.litmus tests act as expected.
And thank you!!!
Thanx, Paul
> have fun, jonas > > > On 1/20/2023 1:34 PM, Jonas Oberhauser wrote: > > I just realized I made a mistake in my earlier response to this message; > > you still need the rf for passing the cookie across threads. > > Perhaps it's better to just also exclude srcu_unlock type events > > explicitly here. > > > > +let srcu-rscs = ([Srcu-lock] ; (data ; [~ Srcu-unlock] ; rf) + ; > > [Srcu-unlock]) & loc > > > > > > best wishes, > > jonas > > > > 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.) > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-2.litmus > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-3.litmus > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-4.litmus > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-5.litmus > > > > > > > > > All as expected. > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-6.litmus > > > > > > > > > Get "Flag unbalanced-srcu-lock" and "Flag unbalanced-srcu-unlock", > > > but this is srcu_down_read() and srcu_up_read(), where this should > > > be OK. Ah, but I need to do the release/acquire/filter > > > trick. Once > > > I did that, it works as expected. > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-7.litmus > > > > > > https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-8.litmus > > > > > > > > > Both as expected. > > > > > > Getting there!!! > > > > > > I also started a regression test, hopefully without pilot error. :-/ > > > > > > Thanx, Paul > > >
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