The pattern here is to avoid possible false sharing. However, due to compiler optimizations the code may simply collapse to the write if we omit READ_ONCE/WRITE_ONCE: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance ================================================================== BUG: KCSAN: data-race in wbt_wait / wbt_wait read to 0xffff88821aa6d140 of 8 bytes by task 10372 on cpu 1: wb_timestamp block/blk-wbt.c:88 [inline] wb_timestamp block/blk-wbt.c:83 [inline] wbt_wait+0x1f9/0x250 block/blk-wbt.c:587 __rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72 rq_qos_throttle block/blk-rq-qos.h:185 [inline] blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971 generic_make_request block/blk-core.c:1064 [inline] generic_make_request+0x196/0x740 block/blk-core.c:1006 submit_bio+0x96/0x3c0 block/blk-core.c:1190 submit_bh_wbc+0x40f/0x460 fs/buffer.c:3095 submit_bh fs/buffer.c:3101 [inline] __bread_slow fs/buffer.c:1177 [inline] __bread_gfp+0xe7/0x1e0 fs/buffer.c:1359 sb_bread include/linux/buffer_head.h:307 [inline] fat__get_entry+0x35e/0x4f0 fs/fat/dir.c:100 fat_get_entry fs/fat/dir.c:128 [inline] fat_get_short_entry+0x103/0x200 fs/fat/dir.c:877 fat_subdirs+0x6b/0x110 fs/fat/dir.c:943 fat_read_root fs/fat/inode.c:1416 [inline] fat_fill_super+0x1552/0x1f50 fs/fat/inode.c:1862 vfat_fill_super+0x3b/0x50 fs/fat/namei_vfat.c:1050 mount_bdev+0x262/0x2d0 fs/super.c:1415 vfat_mount+0x3e/0x60 fs/fat/namei_vfat.c:1057 write to 0xffff88821aa6d140 of 8 bytes by task 10375 on cpu 0: wb_timestamp block/blk-wbt.c:89 [inline] wb_timestamp block/blk-wbt.c:83 [inline] wbt_wait+0x21e/0x250 block/blk-wbt.c:587 __rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72 rq_qos_throttle block/blk-rq-qos.h:185 [inline] blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971 generic_make_request block/blk-core.c:1064 [inline] generic_make_request+0x196/0x740 block/blk-core.c:1006 submit_bio+0x96/0x3c0 block/blk-core.c:1190 mpage_bio_submit fs/mpage.c:66 [inline] mpage_readpages+0x36c/0x3c0 fs/mpage.c:410 blkdev_readpages+0x36/0x50 fs/block_dev.c:620 read_pages+0xa2/0x2d0 mm/readahead.c:126 __do_page_cache_readahead+0x353/0x390 mm/readahead.c:212 force_page_cache_readahead+0x13a/0x1f0 mm/readahead.c:243 page_cache_sync_readahead+0x1cf/0x1e0 mm/readahead.c:522 generic_file_buffered_read mm/filemap.c:2050 [inline] generic_file_read_iter+0xeb6/0x1440 mm/filemap.c:2323 blkdev_read_iter+0xb2/0xe0 fs/block_dev.c:2010 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 10375 Comm: blkid Not tainted 5.4.0-rc3+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 ================================================================== Reported-by: syzbot+ba8947364367f96fe16b@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Marco Elver <elver@xxxxxxxxxx> --- block/blk-wbt.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/block/blk-wbt.c b/block/blk-wbt.c index 8641ba9793c5..ce281a9007a6 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -85,8 +85,8 @@ static void wb_timestamp(struct rq_wb *rwb, unsigned long *var) if (rwb_enabled(rwb)) { const unsigned long cur = jiffies; - if (cur != *var) - *var = cur; + if (cur != READ_ONCE(*var)) + WRITE_ONCE(*var, cur); } } -- 2.24.0.rc1.363.gb1bccd3e3d-goog
