Oleg,
My bad to take so long to reply. I have recently returned from a long vacation.
在 2024/9/28 1:18, Oleg Nesterov 写道:
> On 09/27, Liao Chang wrote:
>>
>> +int recycle_utask_slot(struct uprobe_task *utask, struct xol_area *area)
>> +{
>> + int slot = UINSNS_PER_PAGE;
>> +
>> + /*
>> + * Ensure that the slot is not in use on other CPU. However, this
>> + * check is unnecessary when called in the context of an exiting
>> + * thread. See xol_free_insn_slot() called from uprobe_free_utask()
>> + * for more details.
>> + */
>> + if (test_and_put_task_slot(utask)) {
>> + list_del(&utask->gc);
>> + clear_bit(utask->insn_slot, area->bitmap);
>> + atomic_dec(&area->slot_count);
>> + utask->insn_slot = UINSNS_PER_PAGE;
>> + refcount_set(&utask->slot_ref, 1);
>
> This lacks a barrier, CPU can reorder the last 2 insns
>
> refcount_set(&utask->slot_ref, 1);
> utask->insn_slot = UINSNS_PER_PAGE;
>
> so the "utask->insn_slot == UINSNS_PER_PAGE" check in xol_get_insn_slot()
> can be false negative.
Good catcha! Would an atomic_set() with release ordering be sufficient here
instead of smp_mb()?
>
>> +static unsigned long xol_get_insn_slot(struct uprobe_task *utask,
>> + struct uprobe *uprobe)
>> {
>> struct xol_area *area;
>> unsigned long xol_vaddr;
>> @@ -1665,16 +1740,46 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
>> if (!area)
>> return 0;
>>
>> - xol_vaddr = xol_take_insn_slot(area);
>> - if (unlikely(!xol_vaddr))
>> + /*
>> + * The racing on the utask associated slot_ref can occur unless the
>> + * area runs out of slots. This isn't a common case. Even if it does
>> + * happen, the scalability bottleneck will shift to another point.
>> + */
>
> I don't understand the comment, I guess it means the race with
> recycle_utask_slot() above.
> >> + if (!test_and_get_task_slot(utask))
Exactly, While introducing another refcount operation here might seem like
a downside, the potential racing on it should be less than the ones on
xol_area->bitmap and xol_area->slot_count(which you've already optimized).
>> return 0;
>
> No, we can't do this. xol_get_insn_slot() should never fail.
>
> OK, OK, currently xol_get_insn_slot() _can_ fail, but only if get_xol_area()
> fails to allocate the memory. Which should "never" happen and we can do nothing
> in this case anyway.
Sorry, I haven't trace the exact path where xol_get_insn_slot() fails. I suspect
it might repeatedly trigger BRK exceptions before get_xol_area() successfully
returns. Please correct me if I am wrong.
>
> But it certainly must not fail if it races with another thread, this is insane.
Agreed, it is somewhat costly when race fails. I suggest that it allocates a new
slot upon the race fails instead of returning 0.
>
> And. This patch changes the functions which ask for cleanups. I'll try to send
> a couple of simple patches on Monday.
Thank you for pointing that out, I must have missed some patches while I was on
vacation, I will carefully review the mailing list to ensure that this patch can
work with any recent cleanups.
>
> Oleg.
>
>
--
BR
Liao, Chang
