Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Alan Stern
2023-01-20T16:14:11Z

On Thu, Jan 19, 2023 at 07:55:21PM -0800, Paul E. McKenney wrote:
> 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.)

Jonas is right about the reason for this. Also, his suggestion for
fixing the check in lock.cat makes sense.

My revised patch is 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!!!

Good news.

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 +53,31 @@ 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 carry-srcu-data = (data ; [~ Srcu-unlock] ; rf)*
+let srcu-rscs = ([Srcu-lock] ; carry-srcu-data ; data ; [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); }

Index: usb-devel/tools/memory-model/lock.cat
===================================================================
--- usb-devel.orig/tools/memory-model/lock.cat
+++ usb-devel/tools/memory-model/lock.cat
@@ -36,9 +36,9 @@ let RU = try RU with emptyset
(* Treat RL as a kind of LF: a read with no ordering properties *)
let LF = LF | RL

-(* There should be no ordinary R or W accesses to spinlocks *)
-let ALL-LOCKS = LKR | LKW | UL | LF | RU
-flag ~empty [M \ IW] ; loc ; [ALL-LOCKS] as mixed-lock-accesses
+(* There should be no ordinary R or W accesses to spinlocks or SRCU structs *)
+let ALL-LOCKS = LKR | LKW | UL | LF | RU | Srcu-lock | Srcu-unlock | Sync-srcu
+flag ~empty [M \ IW \ ALL-LOCKS] ; loc ; [ALL-LOCKS] as mixed-lock-accesses

(* Link Lock-Reads to their RMW-partner Lock-Writes *)
let lk-rmw = ([LKR] ; po-loc ; [LKW]) \ (po ; po)