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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>
This commit is contained in:
Paolo Bonzini 2017-01-29 12:15:15 +01:00
parent 43d70ddf9f
commit a70fe14b7d
1 changed files with 2 additions and 2 deletions
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cpu-exec.c
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@ -552,11 +552,11 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
* have set something else (eg exit_request or * have set something else (eg exit_request or
* interrupt_request) which we will handle * interrupt_request) which we will handle
* next time around the loop. But we need to * 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 * comes before the next read of cpu->exit_request
* or cpu->interrupt_request. * or cpu->interrupt_request.
*/ */
smp_rmb(); smp_mb();
*last_tb = NULL; *last_tb = NULL;
break; break;
case TB_EXIT_ICOUNT_EXPIRED: case TB_EXIT_ICOUNT_EXPIRED: