Messages in this thread | | | Date | Fri, 20 Jan 2023 07:39:09 -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 10:43:10AM +0100, Jonas Oberhauser wrote: > > > On 1/19/2023 10:53 PM, 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. > > This sounds like good style. > I suppose this is already flagged as mismatched srcu_unlock(), in case you > accidentally read from the initial write?
It might, except that a filter clause excludes this case. Here is the updated test:
C C-srcu-nest-6
(* * Result: Never * * This would be valid for srcu_down_read() and srcu_up_read(). *)
{}
P0(int *x, int *y, struct srcu_struct *s1, int *idx, int *f) { int r2; int r3;
r3 = srcu_down_read(s1); WRITE_ONCE(*idx, r3); r2 = READ_ONCE(*y); smp_store_release(f, 1); }
P1(int *x, int *y, struct srcu_struct *s1, int *idx, int *f) { int r1; int r3; int r4;
r4 = smp_load_acquire(f); 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] filter (1:r4=1) exists (1:r1=1 /\ 0:r2=0)
> > > 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. > > Agreed. I for one would definitely have something to say about an > > SRCU-usage patch that directly manipulated a srcu_struct structure! ;-) > > Wouldn't the point of having it being flagged be that herd (or similar > tools) would be having something to say long before it has to reach your > pair of eyes?
That would of course be even better.
> I don't think Boqun's patch is hard to repair. > Besides the issue you mention, I think it's also missing Sync-srcu, which > seems to be linked by loc based on its first argument. > > How about something like this? > > let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock | > Sync-srcu flag ~empty ~[ALL_LOCKS | IW] ; loc ; [ALL-LOCKS] as > mixed-lock-accesses > > If you're using something that isn't a lock or intial write on the same location as a lock, you get the flag.
Wouldn't that unconditionally complain about the first srcu_read_lock() in a given process? Or am I misreading those statements?
Thanx, Paul
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