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From: Halil Pasic <pasic@linux.ibm.com>
To: Matthew Rosato <mjrosato@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	qemu-s390x@nongnu.org, qemu-devel@nongnu.org
Cc: Halil Pasic <pasic@linux.ibm.com>, Thomas Huth <thuth@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Richard Henderson <rth@twiddle.net>
Subject: [PATCH 2/2] s390x/pci: fix set_ind_atomic
Date: Tue, 16 Jun 2020 06:50:35 +0200
Message-ID: <20200616045035.51641-3-pasic@linux.ibm.com> (raw)
In-Reply-To: <20200616045035.51641-1-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
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(-)

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,
-- 
2.17.1



  parent reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-16  4:50 [PATCH 0/2] two atomic_cmpxchg() related fixes Halil Pasic
2020-06-16  4:50 ` [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic Halil Pasic
2020-06-16  5:58   ` Christian Borntraeger
2020-06-16  6:33     ` Cornelia Huck
2020-06-16  6:45       ` Christian Borntraeger
2020-06-19  7:14         ` Cornelia Huck
2020-06-17 23:56       ` Halil Pasic
2020-06-19  7:33         ` David Hildenbrand
2020-06-19  8:17           ` Halil Pasic
2020-06-16  9:31     ` Halil Pasic
2020-07-01 13:13   ` Christian Borntraeger
2020-07-04 18:34   ` Michael S. Tsirkin
2020-07-06  5:44     ` Christian Borntraeger
2020-07-06 11:19     ` Halil Pasic
2020-06-16  4:50 ` Halil Pasic [this message]
2020-07-01 13:14   ` [PATCH 2/2] s390x/pci: fix set_ind_atomic Christian Borntraeger
2020-07-01 12:01 ` [PATCH 0/2] two atomic_cmpxchg() related fixes Cornelia Huck
2020-07-01 12:06   ` Christian Borntraeger
2020-07-01 13:10     ` Cornelia Huck
2020-07-03 13:37     ` Halil Pasic
2020-07-03 14:03       ` Cornelia Huck
2020-07-02 11:18 ` Cornelia Huck

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