On Tue, 16 Jun 2020 08:45:14 +0200
Christian Borntraeger <borntrae...@de.ibm.com> wrote:
> On 16.06.20 08:33, Cornelia Huck wrote:
> > On Tue, 16 Jun 2020 07:58:53 +0200
> > Christian Borntraeger <borntrae...@de.ibm.com> wrote:
> >
> >> On 16.06.20 06:50, Halil Pasic wrote:
> >>> The atomic_cmpxchg() loop is broken because we occasionally end up with
> >>> old and _old having different values (a legit compiler can generate code
> >>> that accessed *ind_addr again to pick up a value for _old instead of
> >>> using the value of old that was already fetched according to the
> >>> rules of the abstract machine). This means the underlying CS instruction
> >>> may use a different old (_old) than the one we intended to use if
> >>> atomic_cmpxchg() performed the xchg part.
> >>>
> >>> Let us use volatile to force the rules of the abstract machine for
> >>> accesses to *ind_addr. Let us also rewrite the loop so, we that the
> >>> new old is used to compute the new desired value if the xchg part
> >>> is not performed.
> >>>
> >>> Signed-off-by: Halil Pasic <pa...@linux.ibm.com>
> >>> Reported-by: Andre Wild <andre.wi...@ibm.com>
> >>> Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
> >>> ---
> >>> hw/s390x/virtio-ccw.c | 18 ++++++++++--------
> >>> 1 file changed, 10 insertions(+), 8 deletions(-)
> >>>
> >>> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> >>> index c1f4bb1d33..3c988a000b 100644
> >>> --- a/hw/s390x/virtio-ccw.c
> >>> +++ b/hw/s390x/virtio-ccw.c
> >>> @@ -786,9 +786,10 @@ static inline VirtioCcwDevice
> >>> *to_virtio_ccw_dev_fast(DeviceState *d)
> >>> static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> >>> uint8_t to_be_set)
> >>> {
> >>> - uint8_t ind_old, ind_new;
> >>> + uint8_t expected, actual;
> >>> hwaddr len = 1;
> >>> - uint8_t *ind_addr;
> >>> + /* avoid multiple fetches */
> >>> + uint8_t volatile *ind_addr;
> >>>
> >>> ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
> >>> if (!ind_addr) {
> >>> @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch,
> >>> uint64_t ind_loc,
> >>> __func__, sch->cssid, sch->ssid, sch->schid);
> >>> return -1;
> >>> }
> >>> + actual = *ind_addr;
> >>> do {
> >>> - ind_old = *ind_addr;
> >>
> >> to make things easier to understand. Adding a barrier in here also fixes
> >> the issue.
> >> Reasoning follows below:
> >>
> >>> - ind_new = ind_old | to_be_set;
> >>
> >> with an analysis from Andreas (cc)
> >>
> >> #define atomic_cmpxchg__nocheck(ptr, old, new) ({ \
> >>
> >>
> >> typeof_strip_qual(*ptr) _old = (old); \
> >>
> >>
> >> (void)__atomic_compare_exchange_n(ptr, &_old, new, false, \
> >>
> >>
> >> __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \
> >>
> >>
> >> _old; \
> >>
> >>
> >> })
> >>
> >> ind_old is copied into _old in the macro. Instead of doing the copy from
> >> the
> >> register the compiler reloads the value from memory. The result is that
> >> _old
> >> and ind_old end up having different values. _old in r1 with the bits set
> >> already and ind_old in r10 with the bits cleared. _old gets updated by CS
> >> and matches ind_old afterwards - both with the bits being 0. So the !=
> >> compare is false and the loop is left without having set any bits.
> >>
> >>
> >> Paolo (to),
> >> I am asking myself if it would be safer to add a barrier or something like
> >> this in the macros in include/qemu/atomic.h.
>
> Having said this, I think that the refactoring from Halil (to re-use actual)
> also makes sense independent of the fix.
What about adding a barrier instead, as you suggested?
(Still wondering about other users of atomic_cmpxchg(), though.)
