Fast sigblock (AKA rtld speedup)

[Alfred Perlstein]

On 1/7/13 7:08 PM, Konstantin Belousov wrote:
> On Mon, Jan 07, 2013 at 03:42:01PM -0500, Alfred Perlstein wrote:
>> On 1/7/13 1:22 PM, Konstantin Belousov wrote:
>>> Below is the forward of the patch for which I failed to obtain a private
>>> review. Might be, the list generates more responses.
>> Very cool.
>>
>> Sorry for being rusty here, but is it safe to call fuword in the middle
>> of issignal()?
>>
>> The reason I ask is because it looks like proc lock is required for this
>> block, and I forget it fuword(9) can fault.
>>
>> If fuword(9) can fault then I don't think you can do this.
>>
>> You'll need to use something like fuswintr(9) or wire the page down
>> using vslock(9) or something.
>>
>> I am probably missing something?
> No, you are completely right. This is a serious bug. Thank you.
>
> I was careful to code the td_sigblock_val prefetch etc, but failed
> to do the update properly. There is no reason to execute the update
> in the issignal() at all, the setting of the pending bit can be postponed
> to the moment after the postsig() loop was executed.
>
> I uploaded the updated but not yet tested patch at
> http://people.freebsd.org/~kib/misc/rtld-sigblock.1.patch

This new patch looks like it may have issues with PSTOP.

there is a lot of code in issignal() that is missed when this code is in 
place, I have not audited the effect of this, are you sure this is what 
will work for the majority of cases?

Honestly, if I were coding it for correctness I would vslock the pages 
(or otherwise wire them in) and let issignal() behave normally and not 
early exit from it.

That said I may be missing something.

-Alfred