On Thu, Apr 04, 2024 at 03:38:53PM +1100, Rohan McLure wrote: > 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); Why are you changing this store to be unconditional? Will
