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". > 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 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. 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?".
