Messages in this thread | | | Date | Wed, 13 Feb 2019 15:17:12 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region |
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On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote: > > This; how is getting preempted fundamentally different from scheduling > > ourselves? > > The difference is because getting preempted in the sequence above is > triggered off the back of an interrupt. On arm64, and I think also on x86, > the user access state (SMAP or PAN) is saved and restored across exceptions > but not across context switch. Consequently, taking an irq in a > user_access_{begin,end} section and then scheduling is fine, but calling > schedule directly within such a section is not.
So how's this then:
if (user_access_begin()) {
preempt_disable();
<IRQ> set_need_resched(); </IRQ no preempt>
preempt_enable() __schedule();
user_access_end(); }
That _should_ work just fine but explodes with the proposed nonsense.
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