On Wed, Jun 26, 2013 at 11:55:33AM +0100, Ard Biesheuvel wrote:

Replying to self:

On 25 June 2013 22:24, Ard Biesheuvel [off-list ref] wrote:

quoted

+void kernel_neon_end(void)
+{
+       /* Disable the NEON/VFP unit. */
+       fmxr(FPEXC, fmrx(FPEXC) & ~FPEXC_EN);
+       barrier();
+       dec_preempt_count();
+}
+EXPORT_SYMBOL(kernel_neon_end);

Meh. This is not going to please the RT crowd, as preempt_schedule()
will not be called on PREEMPT builds in this case.

Propose to replace it with

    preempt_enable();
#ifndef CONFIG_PREEMPT_COUNT
    /* in this case, the preempt_enable() right above is just a barrier() */
    dec_preempt_count();
#endif

(and the converse in kernel_neon_begin())

Yuck, that's ugly as sin! How does x86 deal with this? Looking at
kernel_fpu_{begin,end}, they just disable preemption so I guess that they
assume the caller is non-blocking? There's an aside about the use of
preempt-notifiers for KVM, so it does sound like the onus is on the caller
not to shoot themselves in the face.

Will