[PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq
From: Eric Biggers <ebiggers@kernel.org>
Date: 2020-07-13 03:35:06
Also in:
linux-fsdevel, linux-xfs
Subsystem:
filesystems (vfs and infrastructure), filesystems [iomap], the rest · Maintainers:
Alexander Viro, Christian Brauner, Linus Torvalds
From: Eric Biggers <redacted> Fix the preliminary checks for ->s_dio_done_wq to use READ_ONCE(), since it's a data race, and technically the behavior is undefined without READ_ONCE(). Also, on one CPU architecture (Alpha), the data read dependency barrier included in READ_ONCE() is needed to guarantee that the pointed-to struct is seen as fully initialized. Signed-off-by: Eric Biggers <redacted> --- fs/direct-io.c | 8 +++----- fs/internal.h | 9 ++++++++- fs/iomap/direct-io.c | 3 +-- 3 files changed, 12 insertions(+), 8 deletions(-)
diff --git a/fs/direct-io.c b/fs/direct-io.c
index 6d5370eac2a8..26221ae24156 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c@@ -590,7 +590,7 @@ static inline int dio_bio_reap(struct dio *dio, struct dio_submit *sdio) * filesystems that don't need it and also allows us to create the workqueue * late enough so the we can include s_id in the name of the workqueue. */ -int sb_init_dio_done_wq(struct super_block *sb) +int __sb_init_dio_done_wq(struct super_block *sb) { struct workqueue_struct *old; struct workqueue_struct *wq = alloc_workqueue("dio/%s",
@@ -615,9 +615,7 @@ static int dio_set_defer_completion(struct dio *dio) if (dio->defer_completion) return 0; dio->defer_completion = true; - if (!sb->s_dio_done_wq) - return sb_init_dio_done_wq(sb); - return 0; + return sb_init_dio_done_wq(sb); } /*
@@ -1250,7 +1248,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode, retval = 0; if (iocb->ki_flags & IOCB_DSYNC) retval = dio_set_defer_completion(dio); - else if (!dio->inode->i_sb->s_dio_done_wq) { + else { /* * In case of AIO write racing with buffered read we * need to defer completion. We can't decide this now,
diff --git a/fs/internal.h b/fs/internal.h
index 9b863a7bd708..6736c9eee978 100644
--- a/fs/internal.h
+++ b/fs/internal.h@@ -178,7 +178,14 @@ extern void mnt_pin_kill(struct mount *m); extern const struct dentry_operations ns_dentry_operations; /* direct-io.c: */ -int sb_init_dio_done_wq(struct super_block *sb); +int __sb_init_dio_done_wq(struct super_block *sb); +static inline int sb_init_dio_done_wq(struct super_block *sb) +{ + /* pairs with cmpxchg() in __sb_init_dio_done_wq() */ + if (likely(READ_ONCE(sb->s_dio_done_wq))) + return 0; + return __sb_init_dio_done_wq(sb); +} /* * fs/stat.c:
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index ec7b78e6feca..dc7fe898dab8 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c@@ -487,8 +487,7 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter, dio_warn_stale_pagecache(iocb->ki_filp); ret = 0; - if (iov_iter_rw(iter) == WRITE && !wait_for_completion && - !inode->i_sb->s_dio_done_wq) { + if (iov_iter_rw(iter) == WRITE && !wait_for_completion) { ret = sb_init_dio_done_wq(inode->i_sb); if (ret < 0) goto out_free_dio;
--
2.27.0