[Qemu-devel] [PATCH v3 1/4] exec: Atomic access to bounce buffer

[Fam Zheng <famz@redhat.com>]

There could be a race condition when two processes call
address_space_map concurrently and both want to use the bounce buffer.

Add an in_use flag in BounceBuffer to sync it.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 exec.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/exec.c b/exec.c
index e97071a..4080044 100644
--- a/exec.c
+++ b/exec.c
@@ -2483,6 +2483,7 @@ typedef struct {
     void *buffer;
     hwaddr addr;
     hwaddr len;
+    bool in_use;
 } BounceBuffer;
 
 static BounceBuffer bounce;
@@ -2571,9 +2572,10 @@ void *address_space_map(AddressSpace *as,
     l = len;
     mr = address_space_translate(as, addr, &xlat, &l, is_write);
     if (!memory_access_is_direct(mr, is_write)) {
-        if (bounce.buffer) {
+        if (atomic_xchg(&bounce.in_use, true)) {
             return NULL;
         }
+        smp_mb();
         /* Avoid unbounded allocations */
         l = MIN(l, TARGET_PAGE_SIZE);
         bounce.buffer = qemu_memalign(TARGET_PAGE_SIZE, l);
@@ -2641,6 +2643,7 @@ void address_space_unmap(AddressSpace *as, void *buffer, hwaddr len,
     qemu_vfree(bounce.buffer);
     bounce.buffer = NULL;
     memory_region_unref(bounce.mr);
+    atomic_mb_set(&bounce.in_use, false);
     cpu_notify_map_clients();
 }
 
-- 
1.9.3