On Tue, May 15, 2018 at 02:45:32PM +0200, Oleg Nesterov wrote: > On 05/15, Peter Zijlstra wrote: > > > > > > Afaict the whole .owner=NULL thing in release already stops the spinners > > > > > > Not really, the new writer will spin in this case, afaics. > > > > > > But this is another problem and probably we do not care. The new writer is > > > almost impossible in this particular case, another freeze_super() should > > > notice frozen != SB_UNFROZEN and return EBUSY. > > > > rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner > > && owner != RWSEM_READER_OWNED, which will fail for !owner. > > Yep. So rwsem_spin_on_owner() goes to "out:" and returns > !rwsem_owner_is_reader() == T. > > IOW, afaics owner == NULL means "spin unconditionally", I guess this is for > the case when the new writer is going to do rwsem_set_owner() or up_write() > has already called rwsem_clear_owner() but didn't do up_write() yet. > > Probably makes sense, but the code is not very clean, Arrgh, you're right... I hate this rwsem code. Some day I'll finish the atomic_long_t version, which similar to mutex, merges the owner and 'count' fields.
