Messages in this thread Patch in this message | | | Date | Mon, 02 Feb 2015 14:43:22 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH 4/6] x86,fpu: use disable_task_lazy_fpu_restore helper |
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On 02/02/2015 02:21 PM, Oleg Nesterov wrote: > I'll try to read this patch tomorrow. Too late for me. > > I think it is fine, but > > On 02/02, riel@redhat.com wrote: >> >> This also fixes the lazy FPU restore disabling in drop_fpu, >> which only really works when !use_eager_fpu(). ... >> >> --- a/arch/x86/include/asm/fpu-internal.h +++ >> b/arch/x86/include/asm/fpu-internal.h @@ -396,7 +396,7 @@ static >> inline void drop_fpu(struct task_struct *tsk) * Forget >> coprocessor state.. */ preempt_disable(); - >> tsk->thread.fpu_counter = 0; + >> task_disable_lazy_fpu_restore(tsk); __drop_fpu(tsk); >> clear_used_math(); > > perhaps this makes sense anyway, but I am not sure if the changelog > is right. > > Note that we clear PF_USED_MATH, last_fpu/has_fpu have no effect > after this.
There are several code paths, including signal handler stack setup and teardown, where we first clear PF_USED_MATH, but later on set it again, after setting up a new math state for the task.
We need to ensure we always use that new math state, and never lazily re-use what is still in the FPU registers.
>> preempt_enable(); @@ -440,7 +440,7 @@ static inline fpu_switch_t >> switch_fpu_prepare(struct task_struct *old, struct ta >> new->thread.fpu_counter > 5); if (__thread_has_fpu(old)) { if >> (!__save_init_fpu(old)) - old->thread.fpu.last_cpu = ~0; + >> task_disable_lazy_fpu_restore(old); else old->thread.fpu.last_cpu >> = cpu; old->thread.fpu.has_fpu = 0; /* But leave fpu_owner_task! >> */ @@ -454,7 +454,7 @@ static inline fpu_switch_t >> switch_fpu_prepare(struct task_struct *old, struct ta stts(); } >> else { old->thread.fpu_counter = 0; - old->thread.fpu.last_cpu = >> ~0; + task_disable_lazy_fpu_restore(old); > > I am also wondering if we can remove this > task_disable_lazy_fpu_restore... I mean, perhaps we should shift > this into __thread_fpu_end() path. But this is almost off-topic and > in any case this patch should not do this.
Good question. I also wonder why this last_cpu = ~0 is there anyway. If !__thread_has_fpu(old), then I don't see how the lazy restore code would ever give a false positive on that CPU.
Either nobody else used the FPU and the task's state is still there, or somebody else did, and the fpu_owner_task will be pointing elsewhere.
It may be possible to simply remove this one completely, but like you said, that should probably be a different patch.
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