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>
Tested-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
---
Moving from linuxppc-dev to broader LKML as it affects all
architectures.
https://lore.kernel.org/all/20230510033117.1395895-4-rmclure@xxxxxxxxxxxxx/
---
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)
--
2.37.2
