[dpdk-dev] [PATCH] vhost: make the page logging atomic

[Tiwei Bie]

Each dirty page logging operation should be atomic. But it's not
atomic in current implementation. So it's possible that some dirty
pages can't be logged successfully when different threads try to
log different pages into the same byte of the log buffer concurrently.
This patch fixes this issue.

Fixes: b171fad1ffa5 ("vhost: log used vring changes")
Cc: stable at dpdk.org

Reported-by: Xiao Wang <xiao.w.wang at intel.com>
Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
---
 lib/librte_vhost/vhost.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 0f294f3..6fe72ae 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -201,10 +201,19 @@ struct virtio_net {
 
 #define VHOST_LOG_PAGE	4096
 
+/*
+ * Atomically set a bit in memory.
+ */
+static __rte_always_inline void
+vhost_set_bit(unsigned int nr, volatile uint8_t *addr)
+{
+	__sync_fetch_and_or_8(addr, (1U << nr));
+}
+
 static __rte_always_inline void
 vhost_log_page(uint8_t *log_base, uint64_t page)
 {
-	log_base[page / 8] |= 1 << (page % 8);
+	vhost_set_bit(page % 8, &log_base[page / 8]);
 }
 
 static __rte_always_inline void
-- 
2.7.4