[PATCH v2 3/3] accel/tcg: Take mmap_lock in load_atomic*_or_exit

[Richard Henderson]

For user-only, the probe for page writability may race with another
thread's mprotect.  Take the mmap_lock around the operation.  This
is still faster than the start/end_exclusive fallback.

Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
---
 accel/tcg/ldst_atomicity.c.inc | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/accel/tcg/ldst_atomicity.c.inc b/accel/tcg/ldst_atomicity.c.inc
index de70531a7a..e5c590a499 100644
--- a/accel/tcg/ldst_atomicity.c.inc
+++ b/accel/tcg/ldst_atomicity.c.inc
@@ -159,9 +159,11 @@ static uint64_t load_atomic8_or_exit(CPUArchState *env, 
uintptr_t ra, void *pv)
      * another process, because the fallback start_exclusive solution
      * provides no protection across processes.
      */
-    if (!page_check_range(h2g(pv), 8, PAGE_WRITE_ORG)) {
-        uint64_t *p = __builtin_assume_aligned(pv, 8);
-        return *p;
+    WITH_MMAP_LOCK_GUARD() {
+        if (!page_check_range(h2g(pv), 8, PAGE_WRITE_ORG)) {
+            uint64_t *p = __builtin_assume_aligned(pv, 8);
+            return *p;
+        }
     }
 #endif
 
@@ -186,25 +188,27 @@ static Int128 load_atomic16_or_exit(CPUArchState *env, 
uintptr_t ra, void *pv)
         return atomic16_read_ro(p);
     }
 
-#ifdef CONFIG_USER_ONLY
     /*
      * We can only use cmpxchg to emulate a load if the page is writable.
      * If the page is not writable, then assume the value is immutable
      * and requires no locking.  This ignores the case of MAP_SHARED with
      * another process, because the fallback start_exclusive solution
      * provides no protection across processes.
+     *
+     * In system mode all guest pages are writable.  For user mode,
+     * we must take mmap_lock so that the query remains valid until
+     * the write is complete -- tests/tcg/multiarch/munmap-pthread.c
+     * is an example that can race.
      */
-    if (!page_check_range(h2g(p), 16, PAGE_WRITE_ORG)) {
-        return *p;
-    }
+    WITH_MMAP_LOCK_GUARD() {
+#ifdef CONFIG_USER_ONLY
+        if (!page_check_range(h2g(p), 16, PAGE_WRITE_ORG)) {
+            return *p;
+        }
 #endif
-
-    /*
-     * In system mode all guest pages are writable, and for user-only
-     * we have just checked writability.  Try cmpxchg.
-     */
-    if (HAVE_ATOMIC128_RW) {
-        return atomic16_read_rw(p);
+        if (HAVE_ATOMIC128_RW) {
+            return atomic16_read_rw(p);
+        }
     }
 
     /* Ultimate fallback: re-execute in serial context. */
-- 
2.34.1