From d60af028c5227de2918640d6ed1f0e37351a4294 Mon Sep 17 00:00:00 2001 From: Sergey Fedorov Date: Fri, 23 Feb 2018 21:32:44 -0500 Subject: [PATCH] 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. Backports commit 9962c478b153a18fe88a6509fe58cd178aff8abc from qemu --- qemu/include/exec/exec-all.h | 29 ++++++++++++++++------------- 1 file changed, 16 insertions(+), 13 deletions(-) diff --git a/qemu/include/exec/exec-all.h b/qemu/include/exec/exec-all.h index 2ef0211a..55272ccc 100644 --- a/qemu/include/exec/exec-all.h +++ b/qemu/include/exec/exec-all.h @@ -366,20 +366,23 @@ 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,