| Message ID | 20200616045035.51641-3-pasic@linux.ibm.com |
|---|---|
| State | New |
| Headers | show |
| Series | two atomic_cmpxchg() related fixes | expand |
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 <pasic@linux.ibm.com> > Fixes: 8cba80c3a0 ("s390: Add PCI bus support") > Reported-by: Christian Borntraeger <borntraeger@de.ibm.com> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com> > --- > hw/s390x/s390-pci-bus.c | 16 +++++++++------- > 1 file changed, 9 insertions(+), 7 deletions(-) > > diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c > index c4a4259f0c..80eb957a91 100644 > --- a/hw/s390x/s390-pci-bus.c > +++ b/hw/s390x/s390-pci-bus.c > @@ -637,22 +637,24 @@ static AddressSpace *s390_pci_dma_iommu(PCIBus *bus, void *opaque, int devfn) > > static uint8_t set_ind_atomic(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) { > s390_pci_generate_error_event(ERR_EVENT_AIRERR, 0, 0, 0, 0); > 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); > - cpu_physical_memory_unmap(ind_addr, len, 1, len); > + expected = actual; > + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); > + } while (actual != expected); > + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); > > - return ind_old; > + return actual; > } > > static void s390_msi_ctrl_write(void *opaque, hwaddr addr, uint64_t data, >
diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c index c4a4259f0c..80eb957a91 100644 --- a/hw/s390x/s390-pci-bus.c +++ b/hw/s390x/s390-pci-bus.c @@ -637,22 +637,24 @@ static AddressSpace *s390_pci_dma_iommu(PCIBus *bus, void *opaque, int devfn) static uint8_t set_ind_atomic(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) { s390_pci_generate_error_event(ERR_EVENT_AIRERR, 0, 0, 0, 0); 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); - cpu_physical_memory_unmap(ind_addr, len, 1, len); + expected = actual; + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set); + } while (actual != expected); + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len); - return ind_old; + return actual; } static void s390_msi_ctrl_write(void *opaque, hwaddr addr, uint64_t data,
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 <pasic@linux.ibm.com> Fixes: 8cba80c3a0 ("s390: Add PCI bus support") Reported-by: Christian Borntraeger <borntraeger@de.ibm.com> --- hw/s390x/s390-pci-bus.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-)