On Wed, May 10, 2023 at 10:59:05AM +0200, Peter Zijlstra wrote: > On Tue, May 09, 2023 at 04:18:59PM -0400, Kent Overstreet wrote: > > On Tue, May 09, 2023 at 09:31:47PM +0200, Peter Zijlstra wrote: > > > On Tue, May 09, 2023 at 12:56:28PM -0400, Kent Overstreet wrote: > > > > This adds a method to tell lockdep not to check lock ordering within a > > > > lock class - but to still check lock ordering w.r.t. other lock types. > > > > > > > > This is for bcachefs, where for btree node locks we have our own > > > > deadlock avoidance strategy w.r.t. other btree node locks (cycle > > > > detection), but we still want lockdep to check lock ordering w.r.t. > > > > other lock types. > > > > > > > > > > ISTR you had a much nicer version of this where you gave a custom order > > > function -- what happend to that? > > > > Actually, I spoke too soon; this patch and the other series with the > > comparison function solve different problems. > > > > For bcachefs btree node locks, we don't have a defined lock ordering at > > all - we do full runtime cycle detection, so we don't want lockdep > > checking for self deadlock because we're handling that but we _do_ want > > lockdep checking lock ordering of btree node locks w.r.t. other locks in > > the system. > > Have you read the ww_mutex code? If not, please do so, it does similar > things. > > The way it gets around the self-nesting check is by using the nest_lock > annotation, the acquire context itself also has a dep_map for this > purpose. So, spent some time plumbing this through the six lock code and seeing how it'd work. I like it in theory, it's got the right semantics and it would allow for lockdep to check that we're not taking locks with more than one btree_trans in the same thread. Unfortunately, we've got code paths that are meant to be called from contexts that may or may not have a btree_trans - and this is fine right now, because they just use trylock(), but having to plumb nest_lock through is going to make a mess of things. (The relevant codepaths include shrinker callbacks, where we definitely can not just init a new btree_trans, and also the btree node write path which can be kicked off from all sorts of places). Can we go with lockdep_set_no_check_recursion() for now? It's a pretty small addition to the lockdep code.
