tcg: Clarify thread safety check in tb_add_jump()

The check is to make sure that another thread hasn't already done the
same while we were outside of tb_lock. Mention this in a comment.
Signed-off-by: NSergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: NSergey Fedorov <sergey.fedorov@linaro.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

include/exec/exec-all.h

@@ -372,20 +372,23 @@ static inline void tb_set_jmp_target(TranslationBlock *tb,
 static inline void tb_add_jump(TranslationBlock *tb, int n,
                                TranslationBlock *tb_next)
 {
-    /* NOTE: this test is only needed for thread safety */
-    if (!tb->jmp_list_next[n]) {
-        qemu_log_mask_and_addr(CPU_LOG_EXEC, tb->pc,
-                               "Linking TBs %p [" TARGET_FMT_lx
-                               "] index %d -> %p [" TARGET_FMT_lx "]\n",
-                               tb->tc_ptr, tb->pc, n,
-                               tb_next->tc_ptr, tb_next->pc);
-        /* patch the native jump address */
-        tb_set_jmp_target(tb, n, (uintptr_t)tb_next->tc_ptr);
-
-        /* add in TB jmp circular list */
-        tb->jmp_list_next[n] = tb_next->jmp_list_first;
-        tb_next->jmp_list_first = (uintptr_t)tb | n;
+    if (tb->jmp_list_next[n]) {
+        /* Another thread has already done this while we were
+         * outside of the lock; nothing to do in this case */
+        return;
     }
+    qemu_log_mask_and_addr(CPU_LOG_EXEC, tb->pc,
+                           "Linking TBs %p [" TARGET_FMT_lx
+                           "] index %d -> %p [" TARGET_FMT_lx "]\n",
+                           tb->tc_ptr, tb->pc, n,
+                           tb_next->tc_ptr, tb_next->pc);
+
+    /* patch the native jump address */
+    tb_set_jmp_target(tb, n, (uintptr_t)tb_next->tc_ptr);
+
+    /* add in TB jmp circular list */
+    tb->jmp_list_next[n] = tb_next->jmp_list_first;
+    tb_next->jmp_list_first = (uintptr_t)tb | n;
 }
 
 /* GETRA is the true target of the return instruction that we'll execute,