tcg: comment on which functions have to be called with tb_lock held

softmmu requires more functions to be thread-safe, because translation
blocks can be invalidated from e.g. notdirty callbacks.  Probably the
same holds for user-mode emulation, it's just that no one has ever
tried to produce a coherent locking there.

This patch will guide the introduction of more tb_lock and tb_unlock
calls for system emulation.

Note that after this patch some (most) of the mentioned functions are
still called outside tb_lock/tb_unlock.  The next one will rectify this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20161027151030.20863-7-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/exec/exec-all.h

@@ -316,6 +316,7 @@ static inline void tb_set_jmp_target(TranslationBlock *tb,
 
 #endif
 
+/* Called with tb_lock held.  */
 static inline void tb_add_jump(TranslationBlock *tb, int n,
                                TranslationBlock *tb_next)
 {