On Wednesday 09 March 2016 04:01 PM, Peter Zijlstra wrote: > On Wed, Mar 09, 2016 at 11:13:49AM +0100, Peter Zijlstra wrote: >> --- >> Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock() >> >> __clear_bit_unlock() is a special little snowflake. While it carries the >> non-atomic '__' prefix, it is specifically documented to pair with >> test_and_set_bit() and therefore should be 'somewhat' atomic. >> >> Therefore the generic implementation of __clear_bit_unlock() cannot use >> the fully non-atomic __clear_bit() as a default. >> >> If an arch is able to do better; is must provide an implementation of >> __clear_bit_unlock() itself. >> > > FWIW, we could probably undo this if we unified all the spinlock based > atomic ops implementations (there's a whole bunch doing essentially the > same), and special cased __clear_bit_unlock() for that. > > Collapsing them is probably a good idea anyway, just a fair bit of > non-trivial work to figure out all the differences and if they matter > etc.. Indeed I thought about this when we first did the SMP port. The only issue was somewhat more generated code with the hashed spinlocks (vs. my dumb 2 spin locks - which as I see now will also cause false sharing - likely ending up in the same cache line), but I was more of a micro-optimization freak then than I'm now :-) So yeah I agree ! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>