Messages in this thread Patch in this message | | | Date | Fri, 20 Jan 2023 13:34:57 +0100 | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) | From | Jonas Oberhauser <> |
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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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