Messages in this thread | | | Date | Wed, 30 Jun 2021 10:43:53 +0200 | From | Miquel Raynal <> | Subject | Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans |
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Hello,
Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> wrote on Wed, 30 Jun 2021 16:04:17 +0800:
> On 18/6/21 12:09 am, Desmond Cheong Zhi Xi wrote: > > Syzbot reported a circular locking dependency: > > https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb > > > > This happens because of the following lock dependencies: > > > > 1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls > > loop_remove, which then calls del_gendisk; this also happens in > > loop_exit which eventually calls loop_remove) > > > > 2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls > > __blkdev_get, which then calls blktrans_open) > > > > 3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans > > calls __register_blkdev) > > > > 4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls > > loop_probe) > > > > Hence there's an overall dependency of: > > > > loop_ctl_mutex ----------> bdev->bd_mutex > > ^ | > > | | > > | v > > major_names_lock <--------- mtd_table_mutex > > > > We can break this circular dependency by holding mtd_table_mutex only > > for the required critical section in register_mtd_blktrans. This > > avoids the mtd_table_mutex -> major_names_lock dependency. > > > > Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com > > Co-developed-by: Christoph Hellwig <hch@lst.de> > > Signed-off-by: Christoph Hellwig <hch@lst.de> > > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> > > --- > > > > Changes in v1 -> v2: > > > > Break the circular dependency in register_mtd_blktrans instead of blk_request_module, as suggested by Christoph Hellwig. > > > > drivers/mtd/mtd_blkdevs.c | 8 ++------ > > 1 file changed, 2 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c > > index fb8e12d590a1..7d26cfe24d05 100644 > > --- a/drivers/mtd/mtd_blkdevs.c > > +++ b/drivers/mtd/mtd_blkdevs.c > > @@ -528,14 +528,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr) > > if (!blktrans_notifier.list.next) > > 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 +541,12 @@ 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; > > } > > > > Hi maintainers, > > Any chance to review this patch? > > For additional reference, the mtd_table_mutex --> major_names_lock hierarchy that can be removed by this patch also appears in a different lock chain: > https://syzkaller.appspot.com/bug?id=cbf5fe846f14a90f05e10df200b08c57941dc750
I'm fine with the patch, but it came too late in the release cycle so now I'm waiting -rc1 to apply it.
Thanks, Miquèl
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