Commit a70fe14b authored Jan 29, 2017 by Paolo Bonzini

cpu-exec: tighten barrier on TCG_EXIT_REQUESTED



This seems to have worked just fine so far on weakly-ordered
architectures, but I don't see anything that prevents the
reordering from:

    store 1 to exit_request
    store 1 to tcg_exit_req
                                 load tcg_exit_req
                                 store 0 to tcg_exit_req
                                 load exit_request
                                 store 0 to exit_request
    store 1 to exit_request
    store 1 to tcg_exit_req

to this:

    store 1 to exit_request
    store 1 to tcg_exit_req
                                 load tcg_exit_req
                                 load exit_request
    store 1 to exit_request
    store 1 to tcg_exit_req
                                 store 0 to tcg_exit_req
                                 store 0 to exit_request

therefore losing a request.  It's possible that other memory barriers
(e.g. in rcu_read_unlock) are hiding it, but better safe than
sorry.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 43d70ddf

cpu-exec.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -552,11 +552,11 @@ static inline void cpu_loop_exec_tb(CPUState cpu, TranslationBlock tb,
		* have set something else (eg exit_request or
		* interrupt_request) which we will handle
		* next time around the loop. But we need to
		* ensure the tcg_exit_req read in generated code
		* ensure the zeroing of tcg_exit_req (see cpu_tb_exec)
		* comes before the next read of cpu->exit_request
		* or cpu->interrupt_request.
		*/
		smp_rmb();
		smp_mb();
		*last_tb = NULL;
		break;
		case TB_EXIT_ICOUNT_EXPIRED: