Re: [PATCH 05/12] x86-64: Disable preemption when using TS_USEDFPU

[Pekka Enberg]

 On 4.9.2010 4.17, Brian Gerst wrote:
> Consolidates code and fixes the below race for 64-bit.
>
> commit 9fa2f37bfeb798728241cc4a19578ce6e4258f25
> Author: torvalds<torvalds>
> Date:   Tue Sep 2 07:37:25 2003 +0000
>
>      Be a lot more careful about TS_USEDFPU and preemption
>
>      We had some races where we testecd (or set) TS_USEDFPU together
>      with sequences that depended on the setting (like clearing or
>      setting the TS flag in %cr0) and we could be preempted in between,
>      which screws up the FPU state, since preemption will itself change
>      USEDFPU and the TS flag.
>
>      This makes it a lot more explicit: the "internal" low-level FPU
>      functions ("__xxxx_fpu()") all require preemption to be disabled,
>      and the exported "real" functions will make sure that is the case.
>
>      One case - in __switch_to() - was switched to the non-preempt-safe
>      internal version, since the scheduler itself has already disabled
>      preemption.
>
>      BKrev: 3f5448b5WRiQuyzAlbajs3qoQjSobw
>
> Signed-off-by: Brian Gerst<brgerst@xxxxxxxxx>
Acked-by: Pekka Enberg <penberg@xxxxxxxxxx>

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