On Tue, Feb 18, 2020 at 11:04:26PM -0500, Qian Cai wrote: > inode::i_size could be accessed concurently as noticed by KCSAN, > > BUG: KCSAN: data-race in iomap_do_writepage / iomap_write_end > > write to 0xffff8bf68fc0cac0 of 8 bytes by task 7484 on cpu 71: > iomap_write_end+0xea/0x530 > i_size_write at include/linux/fs.h:888 > (inlined by) iomap_write_end at fs/iomap/buffered-io.c:782 > iomap_write_actor+0x132/0x200 > iomap_apply+0x245/0x8a5 > iomap_file_buffered_write+0xbd/0xf0 > xfs_file_buffered_aio_write+0x1c2/0x790 [xfs] > xfs_file_write_iter+0x232/0x2d0 [xfs] > new_sync_write+0x29c/0x3b0 > __vfs_write+0x92/0xa0 > vfs_write+0x103/0x260 > ksys_write+0x9d/0x130 > __x64_sys_write+0x4c/0x60 > do_syscall_64+0x91/0xb05 > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > read to 0xffff8bf68fc0cac0 of 8 bytes by task 5901 on cpu 70: > iomap_do_writepage+0xf4/0x450 > i_size_read at include/linux/fs.h:866 > (inlined by) iomap_do_writepage at fs/iomap/buffered-io.c:1558 > write_cache_pages+0x523/0xb20 > iomap_writepages+0x47/0x80 > xfs_vm_writepages+0xc7/0x100 [xfs] > do_writepages+0x5e/0x130 > __writeback_single_inode+0xd5/0xb20 > writeback_sb_inodes+0x429/0x910 > __writeback_inodes_wb+0xc4/0x150 > wb_writeback+0x47b/0x830 > wb_workfn+0x688/0x930 > process_one_work+0x54f/0xb90 > worker_thread+0x80/0x5f0 > kthread+0x1cd/0x1f0 > ret_from_fork+0x27/0x50 > > Reported by Kernel Concurrency Sanitizer on: > CPU: 70 PID: 5901 Comm: kworker/u257:2 Tainted: G L 5.6.0-rc2-next-20200218+ #2 > Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019 > Workqueue: writeback wb_workfn (flush-254:0) > > The write is protected by exclusive inode::i_rwsem (in > xfs_file_buffered_aio_write()) but the read is not. A shattered value > could introduce a logic bug. Fixed it using a pair of WRITE/READ_ONCE(). If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up being split, the kernel is FUBAR anyway. Details, please - how could that end up happening?
