On Mon, Jan 13, 2025 at 11:07:42PM +0800, Ming Lei wrote: > On Mon, Jan 13, 2025 at 05:49:43AM -0800, Christoph Hellwig wrote: > > On Mon, Jan 13, 2025 at 05:24:46PM +0800, Ming Lei wrote: > > > > Please state the locks. Nothing fs internal here, that report is > > > > about i_rwsem. And a false positive because it is about ordering > > > > of i_rwsem on the upper file system sitting on the loop device vs the > > > > one on the lower file systems sitting below the block device. These > > > > obviously can't deadlock, we just need to tell lockdep about that fact. > > > > > > How can you guarantee that some code won't submit IO by grabbing the > > > i_rwsem? > > > > ? A lot of the I/O will grab i_rwsem on the underlying device. > > Basically all writes, and for many file systems also on reads. But > > that is an entirely different i_rwsem as the one held the bio submitter > > as that is in different file system. There is no way the top file > > system can lock i_rwsem on the lower file system except through the > > loop driver, and that always sits below the freeze protection. Actually some FSs may call kmalloc(GFP_KERNEL) with i_rwsem grabbed, which could call into real deadlock if IO on the loop disk is caused by the kmalloc(GFP_KERNEL). So it is not one false positive. > > > > > As I explained, it is fine to move out vfs_fsync() out of freeze queue. > > > > > > Actually any lock which depends on freeze queue needs to take a careful > > > look, because freeze queue connects too many global/sub-system locks. > > > > For block layer locks: absolutely. For file systems lock: not at all, > > because we're talking about different file systems instances. The only > > exception would be file systems taking global locks in the I/O path, > > but I sincerely hope no one does that. > > Didn't you see the report on fs_reclaim and sysfs root lock? > > https://lore.kernel.org/linux-block/197b07435a736825ab40dab8d91db031c7fce37e.camel@xxxxxxxxxxxxxxx/ There are more, such as mm->mmap_lock[1], hfs SB 'cat_tree' lock[2]... [1] https://lore.kernel.org/linux-block/67863050.050a0220.216c54.006f.GAE@xxxxxxxxxx/ [2] https://lore.kernel.org/linux-block/67582202.050a0220.a30f1.01cb.GAE@xxxxxxxxxx/ Thanks, Ming
