Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 12 Mar 2020 09:07:49 -0700 | Subject | Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression |
| |
On Wed, Mar 11, 2020 at 9:42 PM NeilBrown <neilb@suse.de> wrote: > > It seems that test_and_set_bit_lock() is the preferred way to handle > flags when memory ordering is important
That looks better.
The _preferred_ way is actually the one I already posted: do a "smp_store_release()" to store the flag (like a NULL pointer), and a smp_load_acquire() to load it.
That's basically optimal on most architectures (all modern ones - there are bad architectures from before people figured out that release/acquire is better than separate memory barriers), not needing any atomics and only minimal memory ordering.
I wonder if a special flags value (keeping it "unsigned int" to avoid the issue Jeff pointed out) might be acceptable?
IOW, could we do just
smp_store_release(&waiter->fl_flags, FL_RELEASED);
to say that we're done with the lock? Or do people still look at and depend on the flag values at that point?
Linus
| |