Re: [PATCH] Remove rwsem limitation of 32k waiters

[Matthew Wilcox <matthew@wil.cx>]

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);

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