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SubjectRe: [PATCH] block: break circular locks in blk_request_module
On Thu, Jun 17, 2021 at 05:20:16PM +0800, Desmond Cheong Zhi Xi wrote:
> mutex_lock(&major_names_lock);
> for (n = &major_names[major_to_index(major)]; *n; n = &(*n)->next) {
> if ((*n)->major == major && (*n)->probe) {
> - (*n)->probe(devt);
> + probe = (*n)->probe;
> mutex_unlock(&major_names_lock);
> + probe(devt);

And now you can all probe after it has been freed and/or the module has
been unloaded. The obviously correct fix is to only hold mtd_table_mutex
for the actually required critical section:

diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index fb8e12d590a1..065d94f9b1fb 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -529,13 +529,11 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
register_mtd_user(&blktrans_notifier);


- mutex_lock(&mtd_table_mutex);

ret = register_blkdev(tr->major, tr->name);
if (ret < 0) {
printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
tr->name, tr->major, ret);
- mutex_unlock(&mtd_table_mutex);
return ret;
}

@@ -545,12 +543,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
tr->blkshift = ffs(tr->blksize) - 1;

INIT_LIST_HEAD(&tr->devs);
- list_add(&tr->list, &blktrans_majors);

+ mutex_lock(&mtd_table_mutex);
+ list_add(&tr->list, &blktrans_majors);
mtd_for_each_device(mtd)
if (mtd->type != MTD_ABSENT)
tr->add_mtd(tr, mtd);
-
mutex_unlock(&mtd_table_mutex);
return 0;
}
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