On Mon, Feb 10, 2025 at 03:58:02PM +1100, NeilBrown wrote: > On Sat, 08 Feb 2025, Al Viro wrote: > > 1) what's wrong with using middle bits of dentry as index? What the hell > > is that thing about pid for? > > That does "hell" have to do with it? > > All we need here is a random number. Preferably a cheap random number. > pid is cheap and quite random. > The dentry pointer would be even cheaper (no mem access) providing it > doesn't cost much to get the randomness out. I considered hash_ptr() > but thought that was more code that it was worth. > > Do you have a formula for selecting the "middle" bits in a way that is > expected to still give good randomness? ((unsigned long) dentry / L1_CACHE_BYTES) % <table size> Bits just over the cacheline size should have uniform distribution... > > 2) part in d_add_ci() might be worth a comment re d_lookup_done() coming > > for the original dentry, no matter what. > > I think the previous code deserved explanation more than the new, but > maybe I missed something. > In each case, d_wait_lookup() will wait for the given dentry to no > longer be d_in_lookup() which means waiting for DCACHE_PAR_LOOKUP to be > cleared. The only place which clears DCACHE_PAR_LOOKUP is > __d_lookup_unhash_wake(). which always wakes the target. > In the previous code it would wake both the non-case-exact dentry and > the case-exact dentry waiters but they would go back to sleep if their > DCACHE_PAR_LOOKUP hadn't been cleared, so no interesting behaviour. > Reusing the wq from one to the other is a sensible simplification, but > not something we need any reminder of once it is no longer needed. It's not just about the wakeups; any in-lookup dentry should be taken out of in-lookup hash before it gets dropped. > > 3) the dance with conditional __wake_up() is worth a helper, IMO. I mean an inlined helper function.
