[Qemu-devel] [PATCH v2 5/8] tcg: Clarify thread safety check in tb_add_jump()

[sergey.fedorov@linaro.org]

From: Sergey Fedorov <serge.fdrv@gmail.com>

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: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
---

Notes:
    Changes in v2:
     * Typo fixed in the commit title
     * Complete rewrite of the commit body and the patch based on Paolo's
       comments

 include/exec/exec-all.h | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index db5c658919ad..45142ebab9e2 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -391,16 +391,19 @@ 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]) {
-        /* 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;
-        assert(((uintptr_t)tb & 3) == 0);
-        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;
     }
+
+    /* 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;
+    assert(((uintptr_t)tb & 3) == 0);
+    tb_next->jmp_list_first = (uintptr_t)tb | n;
 }
 
 /* GETRA is the true target of the return instruction that we'll execute,
-- 
2.7.3