[PATCH] asm-generic/mmiowb: Mark accesses to fix KCSAN warnings

[Rohan McLure]

Prior to this patch, data races are detectable by KCSAN of the following
forms:

[1] Asynchronous calls to mmiowb_set_pending() from an interrupt context
    or otherwise outside of a critical section
[2] Interrupted critical sections, where the interrupt will itself
    acquire a lock

In case [1], calling context does not need an mmiowb() call to be
issued, otherwise it would do so itself. Such calls to
mmiowb_set_pending() are either idempotent or no-ops.

In case [2], irrespective of when the interrupt occurs, the interrupt
will acquire and release its locks prior to its return, nesting_count
will continue balanced. In the worst case, the interrupted critical
section during a mmiowb_spin_unlock() call observes an mmiowb to be
pending and afterward is interrupted, leading to an extraneous call to
mmiowb(). This data race is clearly innocuous.

Resolve KCSAN warnings of type [1] by means of READ_ONCE, WRITE_ONCE.
As increments and decrements to nesting_count are balanced by interrupt
contexts, resolve type [2] warnings by simply revoking instrumentation,
with data_race() rather than READ_ONCE() and WRITE_ONCE(), the memory
consistency semantics of plain-accesses will still lead to correct
behaviour.

Signed-off-by: Rohan McLure <rmclure@xxxxxxxxxxxxx>
Reported-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Reported-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
Tested-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
---
Previously discussed here:
https://lore.kernel.org/linuxppc-dev/20230510033117.1395895-4-rmclure@xxxxxxxxxxxxx/
But pushed back due to affecting other architectures. Reissuing, to
linuxppc-dev, as it does not enact a functional change.
---
 include/asm-generic/mmiowb.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/asm-generic/mmiowb.h b/include/asm-generic/mmiowb.h
index 5698fca3bf56..f8c7c8a84e9e 100644
--- a/include/asm-generic/mmiowb.h
+++ b/include/asm-generic/mmiowb.h
@@ -37,25 +37,28 @@ static inline void mmiowb_set_pending(void)
 	struct mmiowb_state *ms = __mmiowb_state();
 
 	if (likely(ms->nesting_count))
-		ms->mmiowb_pending = ms->nesting_count;
+		WRITE_ONCE(ms->mmiowb_pending, ms->nesting_count);
 }
 
 static inline void mmiowb_spin_lock(void)
 {
 	struct mmiowb_state *ms = __mmiowb_state();
-	ms->nesting_count++;
+
+	/* Increment need not be atomic. Nestedness is balanced over interrupts. */
+	data_race(ms->nesting_count++);
 }
 
 static inline void mmiowb_spin_unlock(void)
 {
 	struct mmiowb_state *ms = __mmiowb_state();
+	u16 pending = READ_ONCE(ms->mmiowb_pending);
 
-	if (unlikely(ms->mmiowb_pending)) {
-		ms->mmiowb_pending = 0;
+	WRITE_ONCE(ms->mmiowb_pending, 0);
+	if (unlikely(pending))
 		mmiowb();
-	}
 
-	ms->nesting_count--;
+	/* Decrement need not be atomic. Nestedness is balanced over interrupts. */
+	data_race(ms->nesting_count--);
 }
 #else
 #define mmiowb_set_pending()		do { } while (0)
-- 
2.44.0