Re: [PATCH 1/3] x86, fpu: __kernel_fpu_begin() should clear fpu_owner_task even if use_eager_fpu()

Rik van Riel
2015-01-20T14:16:57Z

On 01/19/2015 01:51 PM, Oleg Nesterov wrote:
> __kernel_fpu_begin() does nothing if !__thread_has_fpu() && use_eager_fpu(),
> perhaps it assumes that this case is simply impossible. This is certainly
> not possible if in_interrupt() == T; interrupted_user_mode() should have
> FPU, and interrupted_kernel_fpu_idle() should fail if !__thread_has_fpu().
>
> However, even if use_eager_fpu() == T a task can do drop_fpu(), then switch
> to another thread which becomes fpu_owner_task, then resume and call some
> function which does kernel_fpu_begin(). Say, an exiting task does a lot of
> things after exit_thread(), it is not safe to assume that it can't use FPU
> in these paths.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Reviewed-by: Rik van Riel <riel@redhat.com>