On Thu, Sep 07, 2023 at 08:20:30PM +0100, Matthew Wilcox wrote: > On Thu, Sep 07, 2023 at 09:08:10PM +0200, Peter Zijlstra wrote: > > On Thu, Sep 07, 2023 at 06:47:01PM +0100, Matthew Wilcox (Oracle) wrote: > > > Several places want to know whether the lock is held by a writer, instead > > > of just whether it's held. We can implement this for both normal and > > > rt rwsems. RWSEM_WRITER_LOCKED is declared in rwsem.c and exposing > > > it outside that file might tempt other people to use it, so just use > > > a comment to note that's what the 1 means, and help anybody find it if > > > they're looking to change the implementation. > > > > I'm presuming this is deep in a callchain where they know they hold the > > lock, but they lost in what capacity? > > No, it's just assertions. You can see that in patch 3 where it's > used in functions called things like "xfs_islocked". Right, but if you're not the lock owner, your answer to the question is a dice-roll, it might be locked, it might not be. > > In general I strongly dislike the whole _is_locked family, because it > > gives very poorly defined semantics if used by anybody but the owner. > > > > If these new functions are indeed to be used only by lock holders to > > determine what kind of lock they hold, could we please put: > > > > lockdep_assert_held() > > > > in them? > > Patch 2 shows it in use in the MM code. We already have a > lockdep_assert_held_write(), but most people don't enable lockdep, so Most devs should run with lockdep on when writing new code, and I know the sanitizer robots run with lockdep on. In general there seems to be a ton of lockdep on coverage. > we also have VM_BUG_ON_MM(!rwsem_is_write_locked(&mm->mmap_lock), mm) > to give us a good assertion when lockdep is disabled. Is that really worth it still? I mean, much of these assertions pre-date lockdep. > XFS has a problem with using lockdep in general, which is that a worker > thread can be spawned and use the fact that the spawner is holding the > lock. There's no mechanism for the worker thread to ask "Does struct > task_struct *p hold the lock?". Will be somewhat tricky to make happen -- but might be doable. It is however an interface that is *very* hard to use correctly. Basically I think you want to also assert that your target task 'p' is blocked, right? That is: assert @p is blocked and holds @lock.
