On Jun 29, 2006, at 9:54 AM, Juergen Hannken-Illjes wrote:

> At least as a base.  Lockmgr locks lack the "do I have a shared  
> lock" query.
> I need a lock where a thread already owning a shared lock succeeds  
> when
> it wants another shared lock.  Last time I looked lockmg locks had  
> this only
> for exclusive locks.

Um, I'm pretty sure that works already with lockmgr locks (e.g.  
recursively acquiring a shared lock you already hold shared).  Just  
make sure the acquire/releases are paired up and you should be fine.

Let's walk through the logic:

         case LK_SHARED:
                 if (WEHOLDIT(lkp, pid, lid, cpu_num) == 0) {

We fall into this case because the lock is held shared (WEHOLDIT()  
returns false).

                         /*
                          * If just polling, check to see if we will  
block.
                          */
                         if ((extflags & LK_NOWAIT) && (lkp->lk_flags &
                             (LK_HAVE_EXCL | LK_WANT_EXCL |  
LK_WANT_UPGRADE))) {
                                 error = EBUSY;
                                 break;
                         }
                         /*
                          * Wait for exclusive locks and upgrades to  
clear.
                          */
                         error = acquire(&lkp, &s, extflags, 0,
                             LK_HAVE_EXCL | LK_WANT_EXCL |  
LK_WANT_UPGRADE);

We wait until no exlusive holders are in / waiting.

                         if (error)
                                 break;
                         lkp->lk_sharecount++;

...and we increment the sharecount.

                         lkp->lk_flags |= LK_SHARE_NONZERO;
                         COUNT(lkp, l, cpu_num, 1);
                         break;
                 }


Looks to me like it should work just fine for what you want to do.

-- thorpej