Re: [PATCH] block: fix deadlock between bd_link_disk_holder and partition scan

[Li Nan]

在 2024/2/8 14:50, Song Liu 写道:
> On Wed, Feb 7, 2024 at 1:32 AM <linan666@xxxxxxxxxxxxxxx> wrote:
> > From: Li Nan <linan122@xxxxxxxxxx>
> >
> > 'open_mutex' of gendisk is used to protect open/close block devices. But
> > in bd_link_disk_holder(), it is used to protect the creation of symlink
> > between holding disk and slave bdev, which introduces some issues.
> >
> > When bd_link_disk_holder() is called, the driver is usually in the process
> > of initialization/modification and may suspend submitting io. At this
> > time, any io hold 'open_mutex', such as scanning partitions, can cause
> > deadlocks. For example, in raid:
> >
> > T1                              T2
> > bdev_open_by_dev
> >   lock open_mutex [1]
> >   ...
> >    efi_partition
> >    ...
> >     md_submit_bio
> >                                  md_ioctl mddev_syspend
> >                                    -> suspend all io
> >                                   md_add_new_disk
> >                                    bind_rdev_to_array
> >                                     bd_link_disk_holder
> >                                      try lock open_mutex [2]
> >      md_handle_request
> >       -> wait mddev_resume
> >
> > T1 scan partition, T2 add a new device to raid. T1 waits for T2 to resume
> > mddev, but T2 waits for open_mutex held by T1. Deadlock occurs.
> >
> > Fix it by introducing a local mutex 'holder_mutex' to replace 'open_mutex'.
>
> Is this to fix [1]? Do we need some Fixes and/or Closes tags?

No. Just use another way to fix [2], and both [2] and this patch can fix
the issue. I am not sure about the root cause of [1] yet.

[2] https://patchwork.kernel.org/project/linux-raid/list/?series=812045

> Could you please add steps to reproduce this issue?

We need to modify the kernel, add sleep in md_submit_bio() and md_ioctl()
as below, and then:
  1. mdadm -CR /dev/md0 -l1 -n2 /dev/sd[bc]  #create a raid
  2. echo 1 > /sys/module/md_mod/parameters/error_inject  #enable sleep
  3. 'mdadm --add /dev/md0 /dev/sda'  #add a disk to raid
  4. submit ioctl BLKRRPART to raid within 10s.


Changes of kernel:
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 350f5b22ba6f..ce16d319edf2 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -76,6 +76,8 @@ static DEFINE_SPINLOCK(pers_lock);

 static const struct kobj_type md_ktype;

+static bool error_inject = false;
+
 struct md_cluster_operations *md_cluster_ops;
 EXPORT_SYMBOL(md_cluster_ops);
 static struct module *md_cluster_mod;
@@ -372,6 +374,8 @@ static bool is_suspended(struct mddev *mddev, struct 
bio *bio)

 void md_handle_request(struct mddev *mddev, struct bio *bio)
 {
+       if (error_inject)
+               ssleep(10);
 check_suspended:
        if (is_suspended(mddev, bio)) {
                DEFINE_WAIT(__wait);
@@ -7752,6 +7756,8 @@ static int md_ioctl(struct block_device *bdev, 
blk_mode_t mode,
                 */
                if (mddev->pers) {
                        mdu_disk_info_t info;
+                       if (error_inject)
+                               ssleep(10);
                        if (copy_from_user(&info, argp, sizeof(info)))
                                err = -EFAULT;
                        else if (!(info.state & (1<<MD_DISK_SYNC)))
@@ -10120,6 +10126,7 @@ module_param_call(start_ro, set_ro, get_ro, NULL, 
S_IRUSR|S_IWUSR);
 module_param(start_dirty_degraded, int, S_IRUGO|S_IWUSR);
 module_param_call(new_array, add_named_array, NULL, NULL, S_IWUSR);
 module_param(create_on_open, bool, S_IRUSR|S_IWUSR);
+module_param(error_inject, bool, S_IRUSR|S_IWUSR);

 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("MD RAID framework");


--
Thanks,
Nan