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From: Duncan Sands <baldrick@wanadoo.fr>
To: linux-kernel@vger.kernel.org
Subject: Use of sti in entry.S question
Date: Thu, 22 May 2003 09:39:13 +0200
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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.