Messages in this thread | | | Date | Thu, 29 Jan 2015 21:45:34 +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 02:40 PM, Oleg Nesterov wrote: > >>> > >>> - 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
Not sure this is enough, but...
> The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end() > needs to be kept from running during unlazy_fpu().
Yes,
> 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.
But we already have kernel_fpu_disable/enable. unlazy_cpu() can use it to avoid the race ?
> I can certainly merge unlazy_fpu() and save_init_fpu() into the > same function, but I am not sure whether or not it should call > __thread_fpu_end() - it looks like that would be desirable in some > cases, but not in others...
I _think_ that we never actually want __thread_fpu_end(), although it doesn't really hurt if !eager. Probably ulazy/save should do
if (!__save_init_fpu()) __thread_fpu_end();
But again, this is minor.
Oleg.
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