Messages in this thread | | | Date | Fri, 6 Mar 2015 15:01:54 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs |
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On 03/06, Ingo Molnar wrote: > > * Oleg Nesterov <oleg@redhat.com> wrote: > > > On 03/06, Ingo Molnar wrote: > > > > > > * Oleg Nesterov <oleg@redhat.com> wrote: > > > > > > > [...] The patch above looks "obviously safe", but perhaps I am > > > > paranoid too much... > > > > > > IMHO your hack above isn't really acceptable, even for a backport. > > > So lets test the patch below (assuming it's the right thing to do) > > > and move forward? > > > > OK, but please note that this patch is not beckportable. If you think > > that -stable doesn't need this fix, then I agree. > > > > If the caller is do_device_not_available(), then we can not enable > > irqs before __thread_fpu_begin() + restore_fpu_checking(). > > > > 1. Preemption in between can destroy ->fpu.state initialized by > > fpu_finit(), __switch_to() will save the live (wrong) FPU state > > again. > > > > 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is > > not nice too. It will do __save_init_fpu() and this overwrites > > ->fpu.state too. > > > > Starting from v4.0 it does kernel_fpu_disable(), but the older kernels > > do not. > > > > Ingo, this code is really horrible and fragile. We need to cleanup it > > step-by-step, imho. > > How about the patch from David Vrabel? That seems to solve the > irq-disable problem too, right?
I wasn't cc'ed, I guess you mean
[PATCHv4] x86, fpu: remove the logic of non-eager fpu mem allocation at the first usage http://marc.info/?l=linux-kernel&m=142564237705311&w=2
Not sure I understand it correctly after the first quick look, but
1. It conflicts with the recent changes in tip/x86/fpu
2. fpu_ini() initializes current->thread.fpu.state. This looks unneeded, the kernel threads no longer have FPU context and do not abuse CPU.
3. I can be easily wrong, but it looks buggy... Note that arch_dup_task_struct() doesn't allocate child->fpu.state if !tsk_used_math(parent).
Add David...
No, I do not think this patch is a good idea. Perhaps I am wrong, but I think we need other changes. And they should start from init_fpu().
Oleg.
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