On Wed, 24 Mar 2010 00:20:25 -0400 Abhinav Duggal <abhinavduggal@xxxxxxxxx> wrote: > Hi all, > It seems like there is a possible race on delalloc_bytes in btrfs in > kernel 2.6.33.Please examine this issue and let me know if this is a > problem or not.The race could occur between stat system call and > writeback kernel thread. > The lines are > fs/btrfs/inode.c 5422 > fs/btrfs/inode.c 1378 > > The stack traces are > > For file line fs/btrfs/inode.c 5422 > > btrfs_getattr+0x141/0x15e > vfs_getattr+0x47/0xf4 > vfs_fstatat+0x43/0x5a > vfs_stat+0x16/0x18 > sys_newstat+0x3d/0x75 > system_call_fastpath+0x16/0x1b > > And for file line fs/btrfs/inode.c 1378 > > btrfs_clear_bit_hook+0xcc1/0x10f2 > clear_state_bit+0x4db/0xa89 > clear_extent_bit+0x9e1/0xc86 > extent_clear_unlock_delalloc+0x8c/0x1d8 > cow_file_range+0x1356/0x1432 > run_delalloc_range+0x263/0x1702 > __extent_writepage+0x2c4/0x15a8 > extent_write_cache_pages.clone.0+0x211/0x33d > extent_writepages+0x243/0x260 > btrfs_writepages+0x87/0x92 > do_writepages+0x1c/0x25 > writeback_single_inode+0xe2/0x24c > writeback_inodes_wb+0x374/0x48d > wb_writeback+0x121/0x1f6 > wb_do_writeback+0x147/0x159 > bdi_writeback_task+0x3a/0xaa > bdi_start_fn+0x63/0xc7 > kthread+0x7a/0x82 > kernel_thread_helper+0x4/0x10 > > The write to delalloc_bytes in the function btrfs_clear_bit_hook is > protected by the spin_lock delalloc_lock whereas the read is not.The > write is not atomic so the read could return some old value. > > I am not sure what would be the implication of this?Is it ok if stat > system call returns some value which is not consistent with rest of > the fields? If not then is it better to make this field as atomic_t? > I am looking for data races in btrfs and I discovered this using some > kind of dynamic analysis. > Thanks a lot. > Yeah you could race, but its getattr. We could add the locking, but its a global lock, and its just to tell the user how big the file is, so its not terribly important. I think it's worth the tradeoff for a little possible inaccuracy vs. creating more lock contention. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
