Subject: Re: SMP API things, take 2
To: Stefan Grefen <>
From: Jason Thorpe <>
List: tech-smp
Date: 07/29/1999 15:38:24
On Fri, 30 Jul 1999 00:21:40 +0200 
 Stefan Grefen <> wrote:

 > I checked the mail and can't find it (besides the 0/1 issue)
 > On PA-Risc a lock must be cache-line sized. You don't want a lock per page
 > or so. HPUX creates a hash table with physical locks. When a spinlock is 
 > initialized it is entered into the hashlist (by address) multiple spinlock
 > share a physical lock. This reduces memory demand by an order of magnitude.
 > Having a definite end on life time helps a lot here (esp. if the lock is 
 > still locked when discarded).

Aha, I see what you are saying now.  So, in this case, locking one thing
might lock N other things, too?  That's unfortunate; it greatly complicates
the case where you want to lock multiple things (using an ordered locking
protocol, of course :-)

My solution was to tell the compiler to align the lock_data (according
to Bill Sommerfeld, the atomic operations will work if 16-byte aligned,
so the compiler can do this with a simple __attribute__ directive).

 > It is also a great help in debugging, without it the cpu-lock will go 
 > astray and its hard to figure out where the locks are lost. This function
 > would be NOP in the normal case, but could complain if a lock is returned
 > locked or just account the cpu lock count.

LOCKDEBUG already has checks for this; if you free an object with an
embedded lock and it is already lock, it complains at you.

        -- Jason R. Thorpe <>