| Message ID | 20200703100650.621212-14-cohuck@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [PULL,01/14] pc-bios: s390x: cio.c cleanup and compile fix | expand |
On Fri, 3 Jul 2020 12:06:49 +0200 Cornelia Huck <cohuck@redhat.com> wrote: > From: Halil Pasic <pasic@linux.ibm.com> > > 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 Michael T. Has pointed out that this sentence is ungrammatical. s/we// would IMHO solve the problem. Can we fix this before it gets merged? > new old is used to compute the new desired value if the xchg part > is not performed. > > Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.") > Reported-by: Andre Wild <Andre.Wild1@ibm.com> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com> > Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com> > Message-Id: <20200616045035.51641-2-pasic@linux.ibm.com> > Signed-off-by: Cornelia Huck <cohuck@redhat.com> > --- > 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 c1f4bb1d331d..3c988a000b5b 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; > - ind_new = ind_old | to_be_set; > - } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old); > - trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new); > - cpu_physical_memory_unmap(ind_addr, len, 1, len); > + expected = actual; > + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); > + } while (actual != expected); > + trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set); > + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); > > - return ind_old; > + return actual; > } > > static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
On Mon, 6 Jul 2020 13:23:11 +0200 Halil Pasic <pasic@linux.ibm.com> wrote: > On Fri, 3 Jul 2020 12:06:49 +0200 > Cornelia Huck <cohuck@redhat.com> wrote: > > > From: Halil Pasic <pasic@linux.ibm.com> > > > > 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 > > Michael T. Has pointed out that this sentence is ungrammatical. > > s/we// would IMHO solve the problem. Can we fix this before it gets > merged? Unfortunately, it's already too late :( > > > new old is used to compute the new desired value if the xchg part > > is not performed.
On Fri, 3 Jul 2020 12:06:49 +0200 Cornelia Huck <cohuck@redhat.com> wrote: > From: Halil Pasic <pasic@linux.ibm.com> > > 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. > [..] I believe this fix should be considered for stable. Unfortunately we didn't think about it back then. Regards, Halil
diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c index c1f4bb1d331d..3c988a000b5b 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; - ind_new = ind_old | to_be_set; - } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old); - trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new); - cpu_physical_memory_unmap(ind_addr, len, 1, len); + expected = actual; + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); + } while (actual != expected); + trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set); + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); - return ind_old; + return actual; } static void virtio_ccw_notify(DeviceState *d, uint16_t vector)