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

Paul E. McKenney
2023-01-20T18:59:58Z

On Fri, Jan 20, 2023 at 01:15:58PM -0500, Alan Stern wrote:
> On Fri, Jan 20, 2023 at 09:30:54AM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 20, 2023 at 11:14:06AM -0500, Alan Stern wrote:
> > > My revised patch is below.
> >
> > Thank you! Are you OK with my putting this on a not-for-mainline branch
> > for experimental purposes?
>
> Sure, go ahead.

And done! This is on -rcu branch lkmm-srcu.2023.01.20a, as shown below.

Thanx, Paul

------------------------------------------------------------------------

commit f0d4b328e12cdc7f34c11f7c82b28a16e097f769
Author: Alan Stern <stern@rowland.harvard.edu>
Date: Fri Jan 20 10:34:59 2023 -0800

tools/memory-model: Provide exact SRCU semantics

LKMM has long provided only approximate handling of SRCU read-side
critical sections. This has not been a pressing problem because LKMM's
traditionall handling is correct for the common cases of non-overlapping
and of properly nested critical sections. However, LKMM's traditional
handling of partially overlapping critical sections incorrectly fuses
them into one large critical section.

For example, consider the following litmus test:

------------------------------------------------------------------------

C C-srcu-nest-5

(*
* Result: Sometimes
*
* This demonstrates non-nested overlapping of SRCU read-side critical
* sections. Unlike RCU, SRCU critical sections do not unconditionally
* nest.
*)

{}

P0(int *x, int *y, struct srcu_struct *s1)
{
int r1;
int r2;
int r3;
int r4;

r3 = srcu_read_lock(s1);
r2 = READ_ONCE(*y);
r4 = srcu_read_lock(s1);
srcu_read_unlock(s1, r3);
r1 = READ_ONCE(*x);
srcu_read_unlock(s1, r4);
}

P1(int *x, int *y, struct srcu_struct *s1)
{
WRITE_ONCE(*y, 1);
synchronize_srcu(s1);
WRITE_ONCE(*x, 1);
}

locations [0:r1]
exists (0:r1=1 /\ 0:r2=0)

------------------------------------------------------------------------

But current mainline incorrectctly flattens the two critical section
into one larger critical section, giving "Never" instead of the correct
"Sometimes":

------------------------------------------------------------------------

$ herd7 -conf linux-kernel.cfg C-srcu-nest-5.litmus
Test C-srcu-nest-5 Allowed
States 3
0:r1=0; 0:r2=0;
0:r1=0; 0:r2=1;
0:r1=1; 0:r2=1;
No
Witnesses
Positive: 0 Negative: 3
Flag srcu-bad-nesting
Condition exists (0:r1=1 /\ 0:r2=0)
Observation C-srcu-nest-5 Never 0 3
Time C-srcu-nest-5 0.01
Hash=e692c106cf3e84e20f12991dc438ff1b

------------------------------------------------------------------------

To its credit, it does complain. But with this commit, we get the
following result, which has the virtue of being correct:

------------------------------------------------------------------------

$ herd7 -conf linux-kernel.cfg C-srcu-nest-5.litmus
Test C-srcu-nest-5 Allowed
States 4
0:r1=0; 0:r2=0;
0:r1=0; 0:r2=1;
0:r1=1; 0:r2=0;
0:r1=1; 0:r2=1;
Ok
Witnesses
Positive: 1 Negative: 3
Condition exists (0:r1=1 /\ 0:r2=0)
Observation C-srcu-nest-5 Sometimes 1 3
Time C-srcu-nest-5 0.05
Hash=e692c106cf3e84e20f12991dc438ff1b

------------------------------------------------------------------------

In addition, there are srcu_down_read() and srcu_up_read() functions on
their way to mainline. Roughly speaking, these are to srcu_read_lock()
and srcu_read_unlock() as mutex_lock() and mutex_unlock() are to down()
and up(). The key point is that srcu_down_read() can execute in one
process and the matching srcu_up_read() in another, as shown in this
litmus 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)

------------------------------------------------------------------------

When run on current mainline, this litmus test gets a complaint about
an unknown macro srcu_down_read(). With this commit:

------------------------------------------------------------------------

herd7 -conf linux-kernel.cfg C-srcu-nest-6.litmus
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 0.02
Hash=c1f20257d052ca5e899be508bedcb2a1

------------------------------------------------------------------------

Note that the user must supply the flag "f" and the "filter" clause,
similar to what must be done to emulate call_rcu().

[ paulmck: Fix space-before-tab whitespace nit. ]

TBD-contributions-from: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Not-yet-signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/tools/memory-model/linux-kernel.bell b/tools/memory-model/linux-kernel.bell
index 70a9073dec3e5..6e702cda15e18 100644
--- a/tools/memory-model/linux-kernel.bell
+++ b/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
diff --git a/tools/memory-model/linux-kernel.def b/tools/memory-model/linux-kernel.def
index ef0f3c1850dee..e1f65e6de06f1 100644
--- a/tools/memory-model/linux-kernel.def
+++ b/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); }

diff --git a/tools/memory-model/lock.cat b/tools/memory-model/lock.cat
index 6b52f365d73ac..53b5a492739d0 100644
--- a/tools/memory-model/lock.cat
+++ b/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)