On Fri, May 18, 2018 at 01:03:39PM +0200, Peter Zijlstra wrote: > On Fri, May 18, 2018 at 06:13:53AM -0400, Kent Overstreet wrote: > > On Fri, May 18, 2018 at 11:51:02AM +0200, Peter Zijlstra wrote: > > > On Fri, May 18, 2018 at 03:49:04AM -0400, Kent Overstreet wrote: > > > > bcachefs makes use of them - also, add a proper lg_lock_init() > > > > > > Why?! lglocks are horrid things, we got rid of them for a reason. They > > > have terrifying worst case preemption off latencies. > > > > Ah. That was missing from your commit message. > > Yeah, sorry, sometimes it's hard to state what is obvious to oneself :/ > > > > Why can't you use something like per-cpu rwsems? > > > > Well, > > > > a) in my use case, lg_global_lock() pretty much isn't used in normal operation, > > it's only called when starting mark and sweep gc (which is not needed > > anymore and disabled by default, it'll eventually get rolled into online > > fsck) and for device resize > > > > b) I'm using it in conjection with percpu counters, and technically yes I > > certainly _could_ use per-cpu sleepable locks (mutexes would make more sense > > for me than rwsems), there's a bit of a clash there and it's going to be a > > bit ugly and messy and it's more work for me. (this_cpu_ptr() no longer > > makes any sense in that case, so it'd mean auditing and converting all the > > code that touches the relevant data structures). > > Well, lg is a reader-writer style lock per definition, as you want > concurrency on the local and full exclusion against the global, so I'm > not sure how mutexes fit into this. > > In any case, have a look at percpu_down_read_preempt_disable() and > percpu_up_read_preempt_enable(); they're a bit of a hack but they should > work for you I think. > > They will sleep at down_read, but the entire actual critical section > will be with preemption disabled -- therefore it had better be short and > bounded, and the RT guys will thank you for not using spinlock_t under > it (use raw_spinlock_t if you have to). > > The (global) writer side will block and be preemptible like normal. > > > If you're really that dead set against lglocks I might just come up with a new > > lock with similar semantics, that doesn't break this_cpu_ptr() but sleeps if the > > global lock is held... > > See above, we already have this ;-) Ok, I think this might work. I'll have to stare awhile and make sure I remember everything I'm currently depending on the lglock for...
