Use of sti in entry.S question

Use of sti in entry.S question

[Duncan Sands]

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:

#ifdef CONFIG_PREEMPT
ENTRY(resume_kernel)
        cmpl $0,TI_PRE_COUNT(%ebp)      # non-zero preempt_count ?
        jnz restore_all
need_resched:
        movl TI_FLAGS(%ebp), %ecx       # need_resched set ?
        testb $_TIF_NEED_RESCHED, %cl
        jz restore_all
        testl $IF_MASK,EFLAGS(%esp)     # interrupts off (exception path) ?
        jz restore_all
        movl $PREEMPT_ACTIVE,TI_PRE_COUNT(%ebp)
        sti <====== schedule with interrupts enabled
        call schedule
        movl $0,TI_PRE_COUNT(%ebp)
        cli
        jmp need_resched
#endif

Thanks,

Duncan.

Another use of sti in entry.S question

[Duncan Sands]

Here is another place where interrupts get turned on in a called
function:

work_resched disables interrupts and falls through to work_notifysig.
work_notifysig calls do_notify_resume.
do_notify_resume calls do_signal.
do_signal calls get_signal_to_deliver.
get_signal_to_deliver does an spin_unlock_irq before returning,
enabling interrupts.

Is this a bug?

work_resched:
        sti
        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

work_notifysig:                         # deal with pending signals and
                                        # notify-resume requests
        testl $VM_MASK, EFLAGS(%esp)
        movl %esp, %eax
        jne work_notifysig_v86          # returning to kernel-space or
                                        # vm86-space
        xorl %edx, %edx
        call do_notify_resume <== may enable interrupts
        jmp restore_all

        ALIGN
work_notifysig_v86:
        pushl %ecx
        call save_v86_state
        popl %ecx
        movl %eax, %esp
        xorl %edx, %edx
        call do_notify_resume <== may enable interrupts
        jmp restore_all