On Thu, Jun 27, 2013 at 02:11:27PM +0100, Ard Biesheuvel wrote:

On 26 June 2013 15:13, Ard Biesheuvel [off-list ref] wrote:

quoted

On 26 June 2013 14:52, Ard Biesheuvel [off-list ref] wrote:

quoted

On 26 June 2013 14:40, Will Deacon [off-list ref] wrote:

quoted

With what we currently have in the kernel, no, I can't think of a better
way. However, I also don't think that smuggling in a back-end hack is a good
idea either. How about we follow x86's lead on this and rely on the caller
not to sleep for the timebeing? Then, separately to this patch series, you
could look at augmenting the scheduler so that schedule_debug can complain
if it encounters a task that is not expected to sleep? That seems like the
right place to fix this problem, and will benefit other architectures too.

Good point. As preempt_enable/disable already have this side effect on
PREEMPT builds, perhaps it wouldn't be such a bad idea to modify them
in the non-PREEMPT case to at least complain if schedule() is invoked
during such a section.

It appears we have this already, but in the non-PREEMPT case, it needs
CONFIG_DEBUG_ATOMIC_SLEEP to be defined.
Let's just rely on preempt_disable() to do the right thing...

OK, just one more question before I respin the next (hopefully final) version:
if a caller does sleep after calling kernel_neon_begin() (and thus
receives no warning if he runs a non-PREEMPT build with
CONFIG_DEBUG_ATOMIC_SLEEP disabled), he will most likely find the
NEON/VFP unit disabled after waking up (as we disable it on a context
switch), so any subsequent NEON instructions will trigger the undef
handler.

Should I perhaps expand the vfp_kmode_exception() function which gets
invoked in this case to be more helpful in identifying this condition?
Currently it just BUG()s on conditions that indicate dependence on
support code, and reports an undefined instruction otherwise.

I don't think you need to worry too much about this. We can enable the debug
option if we want proper debugging and the BUG is a good indicator to go and
investigate a potential problem.

Will