Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

[Peter Zijlstra]

On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:
> +static unsigned long xol_take_insn_slot(struct uprobes_xol_area *area)
> +{
> +       unsigned long slot_addr, flags;
> +       int slot_nr;
> +
> +       do {
> +               spin_lock_irqsave(&area->slot_lock, flags);
> +               slot_nr = find_first_zero_bit(area->bitmap, UINSNS_PER_PAGE);
> +               if (slot_nr < UINSNS_PER_PAGE) {
> +                       __set_bit(slot_nr, area->bitmap);
> +                       slot_addr = area->vaddr +
> +                                       (slot_nr * UPROBES_XOL_SLOT_BYTES);
> +                       atomic_inc(&area->slot_count);
> +               }
> +               spin_unlock_irqrestore(&area->slot_lock, flags);
> +               if (slot_nr >= UINSNS_PER_PAGE)
> +                       xol_wait_event(area);
> +
> +       } while (slot_nr >= UINSNS_PER_PAGE);
> +
> +       return slot_addr;
> +} 

Why isn't a find_first_bit() + set_and_test_bit() not sufficient? That
is, what do you need that lock for?
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