[PATCH] eal: fix data race in hugepage prefault

[Stephen Hemminger]

The prefault step in alloc_seg() reads a value from the hugepage and
writes it back unchanged to force the kernel to commit the backing
page. The read and write were not atomic, which races with concurrent
access to the same physical page from a secondary process attaching
to the hugetlbfs-backed mapping during rte_eal_init().

Replace the non-atomic load+store with a single atomic fetch-or of
zero. This touches the page with an atomic read-modify-write without
changing its contents, eliminating the race while preserving the
original intent of forcing a write fault.

Fixes: 0f1631be24bd ("mem: fix page fault trigger")
Cc: stable at dpdk.org

Reported-by: Michal Sieron <michal.sieron at nokia.com>
Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
---
 .mailmap                     | 1 +
 lib/eal/linux/eal_memalloc.c | 7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/.mailmap b/.mailmap
index 4d26d9c286..07c49eb32f 100644
--- a/.mailmap
+++ b/.mailmap
@@ -1086,6 +1086,7 @@ Michal Mazurek <maz at semihalf.com>
 Michal Michalik <michal.michalik at intel.com>
 Michal Nowak <michal2.nowak at intel.com>
 Michal Schmidt <mschmidt at redhat.com>
+Michal Sieron <michal.sieron at nokia.com>
 Michal Swiatkowski <michal.swiatkowski at intel.com>
 Michal Wilczynski <michal.wilczynski at intel.com>
 Michał Mirosław <michal.miroslaw at atendesoftware.pl> <mirq-linux at rere.qmqm.pl>
diff --git a/lib/eal/linux/eal_memalloc.c b/lib/eal/linux/eal_memalloc.c
index a39bc31c7b..e73a0c11a6 100644
--- a/lib/eal/linux/eal_memalloc.c
+++ b/lib/eal/linux/eal_memalloc.c
@@ -25,6 +25,7 @@
 #include <linux/falloc.h>
 #include <linux/mman.h> /* for hugetlb-related mmap flags */
 
+#include <rte_atomic.h>
 #include <rte_common.h>
 #include <rte_log.h>
 #include <rte_eal.h>
@@ -597,10 +598,10 @@ alloc_seg(struct rte_memseg *ms, void *addr, int socket_id,
 
 	/* we need to trigger a write to the page to enforce page fault and
 	 * ensure that page is accessible to us, but we can't overwrite value
-	 * that is already there, so read the old value, and write itback.
-	 * kernel populates the page with zeroes initially.
+	 * that is already there.
+	 * Use an atomic OR with zero to touch the page without changing its contents.
 	 */
-	*(volatile int *)addr = *(volatile int *)addr;
+	(void)rte_atomic_fetch_or_explicit((int *)addr, 0, rte_memory_order_relaxed);
 
 	iova = rte_mem_virt2iova(addr);
 	if (iova == RTE_BAD_PHYS_ADDR) {
-- 
2.53.0