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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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