A data race occurs when two concurrent data paths potentially access
fuse_conn->num_background simultaneously.
Specifically, fuse_request_end() accesses and modifies ->num_background
while holding the bg_lock, whereas fuse_readahead() reads
->num_background without acquiring any lock beforehand. This potential
data race is flagged by KCSAN:
BUG: KCSAN: data-race in fuse_readahead [fuse] / fuse_request_end [fuse]
read-write to 0xffff8883a6666598 of 4 bytes by task 113809 on cpu 39:
fuse_request_end (fs/fuse/dev.c:318) fuse
fuse_dev_do_write (fs/fuse/dev.c:?) fuse
fuse_dev_write (fs/fuse/dev.c:?) fuse
...
read to 0xffff8883a6666598 of 4 bytes by task 113787 on cpu 8:
fuse_readahead (fs/fuse/file.c:1005) fuse
read_pages (mm/readahead.c:166)
page_cache_ra_unbounded (mm/readahead.c:?)
...
value changed: 0x00000001 -> 0x00000000
Annotated the reader with READ_ONCE() and the writer with WRITE_ONCE()
to avoid such complaint from KCSAN.
Suggested-by: Miklos Szeredi <miklos@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
fs/fuse/dev.c | 6 ++++--
fs/fuse/file.c | 2 +-
2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 3ec8bb5e68ff..8e63dba49eff 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -282,6 +282,7 @@ void fuse_request_end(struct fuse_req *req)
struct fuse_mount *fm = req->fm;
struct fuse_conn *fc = fm->fc;
struct fuse_iqueue *fiq = &fc->iq;
+ unsigned int num_background;
if (test_and_set_bit(FR_FINISHED, &req->flags))
goto put_request;
@@ -301,7 +302,8 @@ void fuse_request_end(struct fuse_req *req)
if (test_bit(FR_BACKGROUND, &req->flags)) {
spin_lock(&fc->bg_lock);
clear_bit(FR_BACKGROUND, &req->flags);
- if (fc->num_background == fc->max_background) {
+ num_background = READ_ONCE(fc->num_background);
+ if (num_background == fc->max_background) {
fc->blocked = 0;
wake_up(&fc->blocked_waitq);
} else if (!fc->blocked) {
@@ -315,7 +317,7 @@ void fuse_request_end(struct fuse_req *req)
wake_up(&fc->blocked_waitq);
}
- fc->num_background--;
+ WRITE_ONCE(fc->num_background, num_background - 1);
fc->active_background--;
flush_bg_queue(fc);
spin_unlock(&fc->bg_lock);
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b57ce4157640..07331889bbf3 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1002,7 +1002,7 @@ static void fuse_readahead(struct readahead_control *rac)
struct fuse_io_args *ia;
struct fuse_args_pages *ap;
- if (fc->num_background >= fc->congestion_threshold &&
+ if (READ_ONCE(fc->num_background) >= fc->congestion_threshold &&
rac->ra->async_size >= readahead_count(rac))
/*
* Congested and only async pages left, so skip the
--
2.43.0
