[PATCH for-8.1] accel/tcg: Take mmap_lock in load_atomic16_or_exit

[Richard Henderson <richard.henderson@linaro.org>]

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.

Remove the write probe in load_atomic8_or_exit.  There we don't have
the same machinery for testing the existance of an 8-byte cmpxchg.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 accel/tcg/ldst_atomicity.c.inc | 54 +++++++++++++++-------------------
 1 file changed, 24 insertions(+), 30 deletions(-)

diff --git a/accel/tcg/ldst_atomicity.c.inc b/accel/tcg/ldst_atomicity.c.inc
index 4de0a80492..e7170f8ba2 100644
--- a/accel/tcg/ldst_atomicity.c.inc
+++ b/accel/tcg/ldst_atomicity.c.inc
@@ -152,19 +152,6 @@ static uint64_t load_atomic8_or_exit(CPUArchState *env, uintptr_t ra, void *pv)
         return load_atomic8(pv);
     }
 
-#ifdef CONFIG_USER_ONLY
-    /*
-     * 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.
-     */
-    if (page_check_range(h2g(pv), 8, PAGE_WRITE_ORG)) {
-        uint64_t *p = __builtin_assume_aligned(pv, 8);
-        return *p;
-    }
-#endif
-
     /* Ultimate fallback: re-execute in serial context. */
     cpu_loop_exit_atomic(env_cpu(env), ra);
 }
@@ -186,25 +173,32 @@ 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.
-     */
-    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.
-     */
+    /* We can only use cmpxchg to emulate a load if the page is writable. */
     if (HAVE_ATOMIC128_RW) {
+#ifdef CONFIG_USER_ONLY
+        /*
+         * 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.
+         * 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.
+         */
+        Int128 r;
+
+        mmap_lock();
+        if (page_get_flags(h2g(p)) & PAGE_WRITE_ORG) {
+            r = atomic16_read_rw(p);
+        } else {
+            r = *p;
+        }
+        mmap_unlock();
+        return r;
+#else
+        /* In system mode all guest pages are host writable. */
         return atomic16_read_rw(p);
+#endif
     }
 
     /* Ultimate fallback: re-execute in serial context. */
-- 
2.34.1