Re: [Lse-tech] Re: RFC: patch to allow lock-free traversal of lists with insertion

Victor Yodaiken
2001-10-10T22:29:28Z

On Thu, Oct 11, 2001 at 07:56:43AM +1000, Keith Owens wrote:
> On Wed, 10 Oct 2001 09:54:36 -0600,
> Victor Yodaiken <yodaiken@fsmlabs.com> wrote:
> >Although I kind of like the idea of
> > normal operation create mess by avoiding synchronization
> > when system seems idle, get BKL, and clean up.
>
> That does not work. A process can read an entry from a list then
> perform an operation that puts the process to sleep. When it wakes up
> again, how can it tell if the list has been changed? How can the

In general you're right, and always its better to
reduce contention than to come up with silly algorithms for
reducing the cost of contention, but if you want to live
dangerously:

reader:
atomic increment read_count
do stuff; skip queue elements marked
as zombie
atomic decrement read_count

writer:
spin lock queue
to delete element
mark element as zombie
unspin

cleanup:
spin lock queue
if(read_count == 0){
get big kernel lock
if(read_count is still 0)
// now nobody will be able to get to queue
clean up queue
unlock kernel
}
unlock spin

So you accomplished making the code harder to read and maintain, slowing
down worst case, and maybe reducing average case read spinlock delay.
For some very carefully selected cases this may be a win, but ...