Re: [PATCH v3 11/28] arm64/sve: Core task context handling

[Dave Martin <Dave dot Martin at arm dot com>]

On Wed, Oct 11, 2017 at 05:15:58PM +0100, Catalin Marinas wrote:
> On Tue, Oct 10, 2017 at 07:38:28PM +0100, Dave P Martin wrote:
> > diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> > index 026a7c7..b1409de 100644
> > --- a/arch/arm64/include/asm/fpsimd.h
> > +++ b/arch/arm64/include/asm/fpsimd.h
> > @@ -72,6 +75,20 @@ extern void sve_load_state(void const *state, u32 const *pfpsr,
> >  			   unsigned long vq_minus_1);
> >  extern unsigned int sve_get_vl(void);
> >  
> > +#ifdef CONFIG_ARM64_SVE
> > +
> > +extern size_t sve_state_size(struct task_struct const *task);
> > +
> > +extern void sve_alloc(struct task_struct *task);
> > +extern void fpsimd_release_thread(struct task_struct *task);
> > +
> > +#else /* ! CONFIG_ARM64_SVE */
> > +
> > +static void __maybe_unused sve_alloc(struct task_struct *task) { }
> > +static void __maybe_unused fpsimd_release_thread(struct task_struct *task) { }
> 
> Nitpick: usually we just add static inline functions here rather than
> __maybe_unused.

Fair enough -- come to think of it I've already converted some other
instances of this at Ard's request.

> 
> > diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> > index 29adab8..4831d28 100644
> > --- a/arch/arm64/include/asm/processor.h
> > +++ b/arch/arm64/include/asm/processor.h
> > @@ -39,6 +47,8 @@
> >  #define FPEXC_IDF	(1 << 7)
> >  
> >  /*
> > + * (Note: in this discussion, statements about FPSIMD apply equally to SVE.)
> > + *
> >   * In order to reduce the number of times the FPSIMD state is needlessly saved
> >   * and restored, we need to keep track of two things:
> >   * (a) for each task, we need to remember which CPU was the last one to have
> > @@ -99,6 +109,287 @@
> >   */
> >  static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
> >  
> > +static void sve_free(struct task_struct *task)
> > +{
> > +	kfree(task->thread.sve_state);
> > +	task->thread.sve_state = NULL;
> > +}
> 
> I think we need a WARN_ON if TIF_SVE is still set here (and the callers
> making sure it is cleared). I haven't checked the code paths via
> fpsimd_release_thread() but wondering what happens if we get an
> interrupt between freeing the state and making the pointer NULL, with
> some context switching in a preemptible kernel.

Having a WARN_ON() here may be a decent way to sanity-check that we
don't ever have sve_state NULL with TIF_SVE set.  This is a lot more
economical than putting a WARN_ON() at each dereference of sve_state
(of which there are quite a few).  sve_free() is also a slow path.

Currently, there are two callsites: sve_set_vector_length(), where we
test_and_clear_tsk_thread_flags(task, TIF_SVE) before calling sve_free();
and fpsimd_release_thread() where we "don't care" because the thread
is dying.

Looking more closely though, is the release_thread() path preemptible?
I can't see anything in the scheduler core to ensure this, nor any
general reason why it should be needed.

In which case preemption during thread exit after sve_free() could
result in a NULL deference in fpsimd_thread_switch().


So, I think my favoured approach is:

sve_release_thread()
{
	local_bh_disable();
	fpsimd_flush_task_state(current);
	clear_thread_flag(TIF_SVE);
	local_bh_enable();

	sve_free();
}

The local_bh stuff is cumbersome here, and could be replaced with
barrier()s to force the order of fpsimd_flusk_task_state() versus
clearing TIF_SVE.  Or should the barrier really be in
fpsimd_flush_task_state()?  Disabling softirqs avoids the need to answer
such questions...


Then:

sve_free(task)
{
	WARN_ON(test_thread_flag(TIF_SVE));

	barrier();
	kfree(task->thread.sve_state);
	tash->thread.sve_state = NULL;
}

I'm assuming here that kfree() can't be called safely from atomic
context, but this is unclear.  I would expect to be able to free
GFP_ATOMIC memory from atomic context (though sve_statue is GFP_KERNEL,
so dunno).

> Alternatively, always clear TIF_SVE here before freeing (also wondering
> whether we should make sve_state NULL before the actual freeing but I
> think TIF_SVE clearing should suffice).

Could do.  I feel that the current placement of the TIF_SVE clearing in
sve_set_vector_length() feels "more natural", but this is a pretty
flimsy argument.  How strongly do you feel about this?

[...]

> > + *  * TIF_SVE set:

[...]

> > + *  * TIF_SVE clear:
> > + *
> > + *    An attempt by the user task to execute an SVE instruction causes
> > + *    do_sve_acc() to be called, which does some preparation and then
> > + *    sets TIF_SVE.
> > + *
> > + *    When stored, FPSIMD registers V0-V31 are encoded in
> > + *    task->fpsimd_state; bits [max : 128] for each of Z0-Z31 are
> > + *    logically zero but not stored anywhere; P0-P15 and FFR are not
> > + *    stored and have unspecified values from userspace's point of
> > + *    view.  For hygiene purposes, the kernel zeroes them on next use,
> > + *    but userspace is discouraged from relying on this.
> > + *
> > + *    task->thread.sve_state does not need to be non-NULL, valid or any
> > + *    particular size: it must not be dereferenced.
> > + *
> > + *  * FPSR and FPCR are always stored in task->fpsimd_state irrespctive of
> > + *    whether TIF_SVE is clear or set, since these are not vector length
> > + *    dependent.
> > + */
> 
> This looks fine, thanks for adding the description.

OK, thanks for checking it.

Cheers
---Dave