On Wed, 15 Apr 2020, Yang Shi wrote: > On Wed, Apr 15, 2020 at 7:04 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > On Mon, 13 Apr 2020, Yang Shi wrote: > > > > > > It looks shmem_uncharge() is just called by __split_huge_page() and > > > collapse_file(). The collapse_file() has acquired xa_lock with irq > > > disabled before acquiring info->lock, so it is safe. > > > __split_huge_page() is called with holding xa_lock with irq enabled, > > > but lru_lock is acquired with irq disabled before acquiring xa_lock. > > > > > > So, it is unnecessary to acquire info->lock with irq disabled in > > > shmem_uncharge(). Can syzbot try the below patch? > > > > But I disagree with the patch below. You're right that IRQ-disabling > > here is unnecessary, given its two callers; but I'm not sure that we > > want it to look different from shmem_charge() and all other info->lock > > takers; and, more importantly, I don't see how removing the redundant > > IRQ-saving below could make it any less liable to deadlock. > > Yes, I realized the patch can't suppress the lockdep splat. But, > actually I didn't understand how this deadlock could happen because > info_lock is acquired with IRQ disabled before acquiring > user_shm_lock. So, interrupt can't come in at all if I didn't miss > anything. I think the story it's trying to tell is this (but, like most of us, I do find Mr Lockdep embarrassingly difficult to understand; and I'm not much good at drawing race diagrams either): CPU0 was in user_shm_unlock(), it's got shmlock_user_lock, then an interrupt comes in. It's an endio kind of interrupt, which goes off to test_clear_page_writeback(), which wants the xa_lock on i_pages. Meanwhile, CPU1 was doing some SysV SHM locking, it's got as far as shmem_lock(), it has acquired info->lock, and goes off to user_shm_lock() which wants shmlock_user_lock. But sadly, CPU2 is splitting a shmem THP, calling shmem_uncharge() that wants info->lock while outer level holds xa_lock on i_pages: with interrupts properly disabled, but that doesn't help. Now, that story doesn't quite hold up as a deadlock, because shmem doesn't use writeback tags; and (unless you set shmem_enabled "force") I don't think there's a way to get shmem THPs in SysV SHM (and are they hole-punchable? maybe through MADV_REMOVE); so it looks like we're talking about different inodes. But lockdep is right to report it, and more thought might arrive at a more convincing scenario. Anyway, easily fixed and best fixed. (But now I think my patch must wait until tomorrow.) Hugh
