Re: Use of sti in entry.S question

[Andi Kleen <ak@suse.de>]

Duncan Sands <baldrick@wanadoo.fr> writes:

> 2.5/arch/i386/kernel/entry.S:
> 
> In work_resched, schedule may be called with
> interrupts off:
> 
> work_resched:
>         call schedule
>         cli                             # make sure we don't miss an interrupt
>                                         # setting need_resched or sigpending
>                                         # between sampling and the iret
>         movl TI_FLAGS(%ebp), %ecx
>         andl $_TIF_WORK_MASK, %ecx      # is there any work to be done other
>                                         # than syscall tracing?
>         jz restore_all
>         testb $_TIF_NEED_RESCHED, %cl
>         jnz work_resched <====== schedule with interrupts disabled
> 
> Is this a mistake or an optimization?  Elsewhere in entry.S, interrupts
> are turned on before calling schedule:

It's a mistake, but a harmless one. The scheduler turns off interrupts
soon itself and the instructions it executes before that don't care.
The only reason it's not recommended to call schedule with interrupts
off is that the scheduler will turn them on again, usually breaking
your critical section. In this case it's ok because the next
instrution is a cli again.

-Andi