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SubjectRe: [patch 2/2] x86, fpu: lazy allocation of FPU area - v5

* Suresh Siddha <suresh.b.siddha@intel.com> wrote:

> asmlinkage void math_state_restore(void)
> {
> struct task_struct *me = current;
> - clts(); /* Allow maths ops (or we recurse) */
>
> - if (!used_math())
> - init_fpu(me);
> + if (!used_math()) {
> + local_irq_enable();
> + /*
> + * does a slab alloc which can sleep
> + */
> + if (init_fpu(me)) {
> + /*
> + * ran out of memory!
> + */
> + do_group_exit(SIGKILL);
> + return;
> + }
> + local_irq_disable();
> + }
> +
> + clts(); /* Allow maths ops (or we recurse) */
> restore_fpu_checking(&me->thread.xstate->fxsave);
> task_thread_info(me)->status |= TS_USEDFPU;
> me->fpu_counter++;

hm, three things:

firstly, the clts is now done _after_ fpu_init() - are you sure that's
OK? We do it in this order so that FINIT [on older cpus] does not fault.

secondly, while i know you were responding to review feedback from
others, but the do_group_exit(SIGKILL) looks quite bad. It's totally
undebuggable to the user - not even a coredump will be generated AFAICS
- and the user has no idea that this all happened due to out-of-memory.
A (forced) SIGBUS is our usual answer to out-of-memory situations. [such
as when a pagetable allocation fails] If you get review feedback that
suggests a crappy solution then please resist it! :-)

thirdly, the irq enable/disable worries me. Can it ever trigger in
kernel code that has irqs off? If it happens when kernel uses the FPU in
irqs-off sections (to do SSE optimized routines, etc.) then enabling
irqs is dangerous - the original callsite had it disabled for a reason.
At minimum we should add a debug check to math_state_restore(),
something like:

WARN_ON_ONCE(!(regs->flags & X86_EFLAGS_IF))

(this means we need to pass regs to math_state_restore())

Ingo


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