Messages in this thread | | | Date | Thu, 29 Jan 2015 22:01:18 +0100 | From | Oleg Nesterov <> | Subject | Re: question about save_xstate_sig() - WHY DOES THIS WORK? |
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On 01/27, Rik van Riel wrote: > > On 01/27/2015 03:27 PM, Rik van Riel wrote: > > On 01/27/2015 02:40 PM, Oleg Nesterov wrote: > > >>>> - Why unlazy_fpu() always does __save_init_fpu() even if > >>>> use_eager_fpu? > >>>> > >>>> and note that in this case __thread_fpu_end() is wrong if > >>>> use_eager_fpu, but fortunately the only possible caller of > >>>> unlazy_fpu() is coredump. fpu_copy() checks use_eager_fpu(). > >>>> > >>>> - Is unlazy_fpu()->__save_init_fpu() safe wrt > >>>> __kernel_fpu_begin() from irq? > > > > It looks like it should be safe, as long as __save_init_fpu() knows > > that the task no longer has the FPU after __kernel_fpu_end(), so it > > does not try to save the kernel FPU state to the user's > > task->thread.fpu.state->xstate > > > > The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end() > > needs to be kept from running during unlazy_fpu(). > > > > This means interrupted_kernel_fpu_idle and/or irq_fpu_usable need > > to check whether preemption is disabled, and lock out > > __kernel_fpu_begin() when preemption is disabled. > > > > It does not look like it currently does that... > > ... and that won't work, because preempt_disable() is a noop > without CONFIG_PREEMPT enabled. Sigh. > > Not sure how to work around that, except by having > __Kernel_fpu_end() always restore the task FPU state, if the > task had the FPU when entering.
This is what it does after
[PATCH 2/3] x86, fpu: don't abuse ->has_fpu in __kernel_fpu_{begin,end}() http://marc.info/?l=linux-kernel&m=142134967718861&w=2
(acked by you and already applied).
But probably I misunderstood you, I do not see how this can help... OK, lets discuss this later.
Oleg.
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