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SubjectRe: [PATCH RT v2] arm64: fpsimd: use a local_lock() in addition to local_bh_disable()
On Wed, 18 Jul 2018 11:12:10 +0200
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> > > @@ -1115,7 +1116,7 @@ void kernel_neon_begin(void)
> > >
> > > BUG_ON(!may_use_simd());
> > >
> > > - local_bh_disable();
> > > + local_lock_bh(fpsimd_lock);
> > >
> > > __this_cpu_write(kernel_neon_busy, true);
> > >
> > > @@ -1129,8 +1130,14 @@ void kernel_neon_begin(void)
> > > fpsimd_flush_cpu_state();
> > >
> > > preempt_disable();
> > > -
> > > - local_bh_enable();
> > > + /*
> > > + * ballance atomic vs !atomic context of migrate_disable().
> > > + * local_lock_bh = get_local_var() + spin_lock_bh (2x migrate_disable)
> > > + */
> > > + migrate_disable();
> > > + migrate_disable();
> > > + migrate_disable();
> > > + local_unlock_bh(fpsimd_lock);
> >
> > This looks fragile.
> >
> > Do we really need to do it 3 times?
>
> Unfortunately yes.

Then we need to find another solution, because this is way too ugly and
as Dave said, fragile to keep.


>
> > Showing my ignorance here, but I'd naively expect that the migrate-
> > disableness changes as follows:
> >
> > /* 0 */
> > local_lock_bh(); /* +3 */
> >
> > ...
> >
> > preempt_disable(); /* +3 */
> >
> > migrate_disable(); /* +4 */
> > migrate_disable(); /* +5 */
> > migrate_disable(); /* +6 */
> >
> > local_unlock_bh(); /* +3 */
> >
> >
> > If so, I don't understand why it's not OK to call migrate_disable()
> > just once, leaving the count at +1 (?)
> >
> > I'm making assumptions about the relationship between preempt_disable()
> > and migrate_disable() here.
>
> Exactly. So local_lock_bh() does +3 which I try to explain why it is 3.

How does local_lock_bh() do a +3 (not seeing it in the code). And
leaking internal implementation of local_lock_bh() into other parts of
the kernel is a no no.

> Now migrate_disable() has two counters: One for atomic context (irq or
> preemption disabled) and on for non-atomic context. The difference is
> that in atomic context migrate_disable() does nothing and in !atomic
> context it does other things which can't be done in atomic context
> anyway (I can explain this in full detail but you may lose interest so I
> keep it short). To update your example:
>
> /* ATOMIC COUNTER | non-ATOMIC COUNTER | change */
> /* 0 | 0 | - */
> local_lock_bh(); /* 0 | 3 | N+3 */
>
> ...
>
> preempt_disable(); /* atomic context */
>
> migrate_disable(); /* 1 | 3 | A+1 */
> migrate_disable(); /* 2 | 3 | A+1 */
> migrate_disable(); /* 3 | 3 | A+1 */
>
> local_unlock_bh(); /* 0 | 3 | A-3 */
>
> /* later */
> preempt_enable(); /* non-atomic context */
> migrate_enable(); /* 0 | 2 | N-1 */
> migrate_enable(); /* 0 | 1 | N-1 */
> migrate_enable(); /* 0 | 0 | N-1 */

If anything, this needs a wrapper. local_lock_preempt_fixup() ?

-- Steve

>
> > > }
> > > EXPORT_SYMBOL(kernel_neon_begin);
> > >
> > > @@ -1154,6 +1161,10 @@ void kernel_neon_end(void)
> > > WARN_ON(!busy); /* No matching kernel_neon_begin()? */
> > >
> > > preempt_enable();
> > > + /* balance migrate_disable(). See kernel_neon_begin() */
> > > + migrate_enable();
> > > + migrate_enable();
> > > + migrate_enable();
> > > }
> > > EXPORT_SYMBOL(kernel_neon_end);
> > >
> > > @@ -1185,9 +1196,7 @@ void __efi_fpsimd_begin(void)
> > > if (!system_supports_fpsimd())
> > > return;
> > >
> > > - WARN_ON(preemptible());
> > > -
> > > - if (may_use_simd()) {
> > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && may_use_simd()) {
> >
> > I suspect this is wrong -- see comments on the commit message.
> >
> > > kernel_neon_begin();
> > > } else {
> >
> > [...]
> >
> > Cheers
> > ---Dave

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