Why do I see sometimes a cli()-sti() pair, and sometimes a save_flags-cli()-restore_flags sequence?
• (RRR) The cli()-sti() pair assumes that interrupts were enabled when execution of the code began, and thus proceeds to reenable them at the end. The save_flags-cli-restore_flags sequence doesn’t make this assumption. Since the interrupt flag is one of the flags saved by save_flags(), it will be correctly restored to its previous state by restore_flags(). This is critical for code that may be called with interrupts either on or off. Using save_flags-cli-restore_flags does incur in a very slight overhead as compared to the cli()-sti) pair, which may be significant for speed critical code (apart from being superfluous if it’s known a priori that the code will never be called with interrupts off). • (REG) Note that on UP systems cli(), sti() and restore_flags() operate immediately. However, on SMP systems, these functions may have to wait for the global IRQ lock (when another CPU has disabled interrupts). Other than this difference, these functions are SMP safe. It is also safe to call c
