Messages in this thread | | | Date | Mon, 25 Jul 2011 14:10:02 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork |
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On 07/25/2011 11:20 AM, Cyrill Gorcunov wrote: > On Mon, Jul 25, 2011 at 02:19:02PM +0400, Cyrill Gorcunov wrote: >> On Mon, Jul 25, 2011 at 10:58:03AM +0100, Ian Campbell wrote: >>> The following series removes the use of a global kernel_eflags variable >>> from the x86_64 ret_from_fork path and (very slightly) merges the 32 and >>> 64 bit version of that code path. >>> >>> kernel_eflags could be made a __read_mostly but actually there is no >>> reason to prefer the value at cpu_init() time to a compile time constant >>> value for the initial eflags after a fork. >>> >>> Ian. >>> >> >> Thanks, Ian! I think noone against this simplification, Peter, Andi? >> >> Cyrill > > Ian, I've missed in first place that you've opened IRQs window _before_ > schedule_tail() call, ie it's not 1:1 code mapping as it was before. > > Note kernel_eflags has IF clear and what we have: the ret_from_fork on > x86-64 happens _only_ inside context_switch call, ie > > schedule (sched.c) > ... > raw_spin_lock_irq > ... > context_switch > switch_to > "jnz ret_from_fork\n\t" > pushq_cfi kernel_eflags(%rip) > popfq_cfi # reset kernel eflags > > ---> irqs are still disabled > > call schedule_tail # rdi: 'prev' task parameter > finish_lock_switch > raw_spin_unlock_irq > > I bet raw_spin_lock_irq at the beginning of the schedule() is set > for a reason and such change is not safe. Though I may be missing > something again... >
This definitely doesn't look "obviously safe" to me. However, does anyone see a problem with unconditionally leaving IF disabled even on 32 bits (I haven't traced all the paths yet), i.e. doing the *opposite* of Ian's patch #2?
-hpa
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