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authorDennis Zhou <dennis@kernel.org>2018-12-05 12:10:26 -0500
committerJens Axboe <axboe@kernel.dk>2018-12-07 22:26:36 -0700
commit0fe061b9f03c27d0370888efc22d4b3ac7af90cf (patch)
tree5cc436fd2fc45eaee9d8796c3ee74635712ea18c /block/bio.c
parent6e0de61107f03c3222550d9b548cd331d31d82d1 (diff)
downloadlinux-0fe061b9f03c27d0370888efc22d4b3ac7af90cf.tar.bz2
blkcg: fix ref count issue with bio_blkcg() using task_css
The bio_blkcg() function turns out to be inconsistent and consequently dangerous to use. The first part returns a blkcg where a reference is owned by the bio meaning it does not need to be rcu protected. However, the third case, the last line, is problematic: return css_to_blkcg(task_css(current, io_cgrp_id)); This can race against task migration and the cgroup dying. It is also semantically different as it must be called rcu protected and is susceptible to failure when trying to get a reference to it. This patch adds association ahead of calling bio_blkcg() rather than after. This makes association a required and explicit step along the code paths for calling bio_blkcg(). In blk-iolatency, association is moved above the bio_blkcg() call to ensure it will not return %NULL. BFQ uses the old bio_blkcg() function, but I do not want to address it in this series due to the complexity. I have created a private version documenting the inconsistency and noting not to use it. Signed-off-by: Dennis Zhou <dennis@kernel.org> Acked-by: Tejun Heo <tj@kernel.org> Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/bio.c')
-rw-r--r--block/bio.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/block/bio.c b/block/bio.c
index 03895cc0d74a..346a7f5cb2dd 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1990,13 +1990,19 @@ int bio_associate_blkcg_from_page(struct bio *bio, struct page *page)
*
* This function takes an extra reference of @blkcg_css which will be put
* when @bio is released. The caller must own @bio and is responsible for
- * synchronizing calls to this function.
+ * synchronizing calls to this function. If @blkcg_css is %NULL, a call to
+ * blkcg_get_css() finds the current css from the kthread or task.
*/
int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css)
{
if (unlikely(bio->bi_css))
return -EBUSY;
- css_get(blkcg_css);
+
+ if (blkcg_css)
+ css_get(blkcg_css);
+ else
+ blkcg_css = blkcg_get_css();
+
bio->bi_css = blkcg_css;
return 0;
}