On Mon, Aug 22, 2005 at 12:38:45PM -0700, Christoph Lameter wrote:
> On Mon, 22 Aug 2005, Matthew Wilcox wrote:
> > Why not just change the BIAS value to 0x0010000? As long as you don't
> > have more than 1024 processors trying to simultaneously acquire the same
> > write lock (and if you do, you have other problems ...), this won't fail
> > and gives you 2^19 waiters.
>
> The same approach was already chosen by s390 and alpha.
>
> Changing the bias value reduces the maximum number of waiting
> processes as you noted. Note "processes" not processors. One processor can
> have multiple processes waiting on semaphores. These are not spinlocks.
No, but you didn't look at the implementation in lib/rwsem.c.
static inline struct rw_semaphore *
rwsem_down_failed_common(struct rw_semaphore *sem,
struct rwsem_waiter *waiter, signed long adjustment)
...
/* we're now waiting on the lock, but no longer actively read-locking */
count = rwsem_atomic_update(adjustment, sem);
struct rw_semaphore fastcall __sched *
rwsem_down_write_failed(struct rw_semaphore *sem)
...
rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
--
"Next the statesmen will invent cheap lies, putting the blame upon
the nation that is attacked, and every man will be glad of those
conscience-soothing falsities, and will diligently study them, and refuse
to examine any refutations of them; and thus he will by and by convince
himself that the war is just, and will thank God for the better sleep
he enjoys after this process of grotesque self-deception." -- Mark Twain
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