| Message ID | 20250113120644.811886-1-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | loop: move vfs_fsync() out of loop_update_dio() | expand |
NAK for the same reason as v1. It's not needed because there is no deadlock, just a false positive due to missing annotations of lock instances, and we need to fsync with a frozen queue to ensure there is no outstanding I/O.
On Mon, Jan 13, 2025 at 05:50:34AM -0800, Christoph Hellwig wrote: > NAK for the same reason as v1. It's not needed because there is no > deadlock, just a false positive due to missing annotations of lock Yes, but annotation should be the last straw. > instances, and we need to fsync with a frozen queue to ensure there > is no outstanding I/O. loop_configure() is on unbound loop, so there isn't outstanding I/O. loop_change_fd() is switching to this new file, so no outstanding I/O on this new file before unfreeze. The other two can only switch to buffered IO, which needn't the fsync. So can you point out anything is wrong? And this way is sort of simplification. Thanks, Ming
On Mon, Jan 13, 2025 at 11:04:51PM +0800, Ming Lei wrote: > > instances, and we need to fsync with a frozen queue to ensure there > > is no outstanding I/O. > > loop_configure() is on unbound loop, so there isn't outstanding I/O. Yeah. > loop_change_fd() is switching to this new file, so no outstanding I/O > on this new file before unfreeze. True. > The other two can only switch to buffered IO, which needn't the fsync. > > So can you point out anything is wrong? > > And this way is sort of simplification. Yeah, I think this is fine in this version. But please update the comment away from the avoiding races to spell out that this is writing out the page cache before starting to use direct I/O.
diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 1ec7417c7f00..be7e20064427 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -205,8 +205,6 @@ static bool lo_can_use_dio(struct loop_device *lo) */ static inline void loop_update_dio(struct loop_device *lo) { - bool dio_in_use = lo->lo_flags & LO_FLAGS_DIRECT_IO; - lockdep_assert_held(&lo->lo_mutex); WARN_ON_ONCE(lo->lo_state == Lo_bound && lo->lo_queue->mq_freeze_depth == 0); @@ -215,10 +213,6 @@ static inline void loop_update_dio(struct loop_device *lo) lo->lo_flags |= LO_FLAGS_DIRECT_IO; if ((lo->lo_flags & LO_FLAGS_DIRECT_IO) && !lo_can_use_dio(lo)) lo->lo_flags &= ~LO_FLAGS_DIRECT_IO; - - /* flush dirty pages before starting to issue direct I/O */ - if ((lo->lo_flags & LO_FLAGS_DIRECT_IO) && !dio_in_use) - vfs_fsync(lo->lo_backing_file, 0); } /** @@ -621,6 +615,9 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev, if (get_loop_size(lo, file) != get_loop_size(lo, old_file)) goto out_err; + /* may work in dio, so flush page cache for avoiding race */ + vfs_fsync(file, 0); + /* and ... switch */ disk_force_media_change(lo->lo_disk); blk_mq_freeze_queue(lo->lo_queue); @@ -1098,6 +1095,9 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode, if (error) goto out_unlock; + /* may work in dio, so flush page cache for avoiding race */ + vfs_fsync(file, 0); + loop_update_dio(lo); loop_sysfs_init(lo);
If vfs_flush() is called with queue frozen, the queue freeze lock may be connected with FS internal lock, and lockdep warning can be triggered because the queue freeze lock is connected with too many global or sub-system locks. Fix the warning by moving vfs_fsync() out of loop_update_dio(): - vfs_fsync() is only needed when switching to dio - only loop_change_fd() and loop_configure() may switch from buffered IO to direct IO, so call vfs_fsync() directly here. This way is safe because either loop is in unbound, or new file isn't attached - for the other two cases of set_status and set_block_size, direct IO can only become off, so no need to call vfs_fsync() Cc: Christoph Hellwig <hch@infradead.org> Reported-by: Kun Hu <huk23@m.fudan.edu.cn> Reported-by: Jiaji Qin <jjtan24@m.fudan.edu.cn> Closes: https://lore.kernel.org/linux-block/359BC288-B0B1-4815-9F01-3A349B12E816@m.fudan.edu.cn/T/#u Signed-off-by: Ming Lei <ming.lei@redhat.com> --- drivers/block/loop.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)