Horton

Thread

Commits

  1. Replace CAS loop with single TAS in ProcArrayGroupClearXid()

  1. pg_atomic_exchange_u32() in ProcArrayGroupClearXid()

    Alexander Korotkov <a.korotkov@postgrespro.ru> — 2018-09-21T17:35:28Z

    Hi!

While investigating ProcArrayGroupClearXid() code I wonder why do we have
this loop instead of plain pg_atomic_exchange_u32() call?  Is it
intentional?

/*
* Now that we've got the lock, clear the list of processes waiting for
* group XID clearing, saving a pointer to the head of the list.  Trying
* to pop elements one at a time could lead to an ABA problem.
*/
while (true)
{
nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
   &nextidx,
   INVALID_PGPROCNO))
break;
}

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

  2. Re: pg_atomic_exchange_u32() in ProcArrayGroupClearXid()

    Amit Kapila <amit.kapila16@gmail.com> — 2018-09-22T06:55:09Z

    On Fri, Sep 21, 2018 at 11:06 PM Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
>
> Hi!
>
> While investigating ProcArrayGroupClearXid() code I wonder why do we have this loop instead of plain pg_atomic_exchange_u32() call?
>

We can use pg_atomic_exchange_u32 instead of a loop.  In fact, clog
code uses pg_atomic_exchange_u32 for the same purpose.  I think it is
better to make the code consistent at both places.  Feel free to
change it, otherwise, I can take care of it in a few days time.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

  3. Re: pg_atomic_exchange_u32() in ProcArrayGroupClearXid()

    Alexander Korotkov <a.korotkov@postgrespro.ru> — 2018-09-22T13:28:21Z

    On Sat, Sep 22, 2018 at 9:55 AM Amit Kapila <amit.kapila16@gmail.com> wrote:

> On Fri, Sep 21, 2018 at 11:06 PM Alexander Korotkov
> <a.korotkov@postgrespro.ru> wrote:
> > While investigating ProcArrayGroupClearXid() code I wonder why do we
> have this loop instead of plain pg_atomic_exchange_u32() call?
>
> We can use pg_atomic_exchange_u32 instead of a loop.  In fact, clog
> code uses pg_atomic_exchange_u32 for the same purpose.  I think it is
> better to make the code consistent at both places.  Feel free to
> change it, otherwise, I can take care of it in a few days time.
>

Thank you for feedback.  Pushed.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company