On Fri, 3 Jul 2020 12:06:50 +0200
Cornelia Huck <coh...@redhat.com> wrote:
> From: Halil Pasic <pa...@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: 8cba80c3a0 ("s390: Add PCI bus support")
> Reported-by: Christian Borntraeger <borntrae...@de.ibm.com>
> Signed-off-by: Halil Pasic <pa...@linux.ibm.com>
> Reviewed-by: Christian Borntraeger <borntrae...@de.ibm.com>
> Message-Id: <20200616045035.51641-3-pa...@linux.ibm.com>
> Signed-off-by: Cornelia Huck <coh...@redhat.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 142e52a8ffdd..736965c9287f 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,
