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SubjectRe: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()


On Tue, 29 Sep 2020, Peter Zijlstra wrote:

> On Mon, Sep 28, 2020 at 02:44:57PM +0200, Lukas Bulwahn wrote:
> > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > index 6b0f4c88b07c..90515c04d90a 100644
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -316,7 +316,7 @@ EXPORT_SYMBOL_GPL(leave_mm);
> >
> > int enable_l1d_flush_for_task(struct task_struct *tsk)
> > {
> > - int cpu, ret = 0, i;
> > + int i;
> >
> > /*
> > * Do not enable L1D_FLUSH_OUT if
> > @@ -329,7 +329,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
> > !static_cpu_has(X86_FEATURE_FLUSH_L1D))
> > return -EINVAL;
> >
> > - cpu = get_cpu();
> > + get_cpu();
> >
> > for_each_cpu(i, &tsk->cpus_mask) {
> > if (cpu_data(i).smt_active == true) {
> > @@ -340,7 +340,7 @@ int enable_l1d_flush_for_task(struct task_struct *tsk)
> >
> > set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
> > put_cpu();
> > - return ret;
> > + return 0;
> > }
>
> If you don't use the return value of get_cpu(), then change it over to
> preempt_{dis,en}able(), but this got me looking at the function, wtf is
> that garbage supposed to do in the first place

I also thought that preempt_{dis,en}able() would do, but thought maybe
Balbir just considered {get,put}_cpu stylistically nicer... so I stayed
with the functions as-is.

>
> What do we need to disable preemption for?
>

I have no clue... not a good premise for touching the code, but I just
wanted to make clang-analyzer happy without modifying any semantics.

Balbir, can you help out here? What was your intent?

> Please explain the desired semantics against sched_setaffinity().
>

I am happy to send a proper v2 once I understand if disabling preemption
is required and the preempt_{dis,en}able() function are preferred to the
{get,put}_cpu functions.

Lukas

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