Re: [PATCH v8 09/12] sysfs: fix deadlock race with module removal

Ming Lei
2021-10-12T00:21:16Z

On Mon, Oct 11, 2021 at 02:25:46PM -0700, Luis Chamberlain wrote:
> On Tue, Oct 05, 2021 at 05:24:18PM +0800, Ming Lei wrote:
> > On Mon, Sep 27, 2021 at 09:38:02AM -0700, Luis Chamberlain wrote:
> > > When driver sysfs attributes use a lock also used on module removal we
> > > can race to deadlock. This happens when for instance a sysfs file on
> > > a driver is used, then at the same time we have module removal call
> > > trigger. The module removal call code holds a lock, and then the
> > > driver's sysfs file entry waits for the same lock. While holding the
> > > lock the module removal tries to remove the sysfs entries, but these
> > > cannot be removed yet as one is waiting for a lock. This won't complete
> > > as the lock is already held. Likewise module removal cannot complete,
> > > and so we deadlock.
> > >
> > > This can now be easily reproducible with our sysfs selftest as follows:
> > >
> > > ./tools/testing/selftests/sysfs/sysfs.sh -t 0027
> > >
> > > This uses a local driver lock. Test 0028 can also be used, that uses
> > > the rtnl_lock():
> > >
> > > ./tools/testing/selftests/sysfs/sysfs.sh -t 0028
> > >
> > > To fix this we extend the struct kernfs_node with a module reference
> > > and use the try_module_get() after kernfs_get_active() is called. As
> > > documented in the prior patch, we now know that once kernfs_get_active()
> > > is called the module is implicitly guarded to exist and cannot be removed.
> > > This is because the module is the one in charge of removing the same
> > > sysfs file it created, and removal of sysfs files on module exit will wait
> > > until they don't have any active references. By using a try_module_get()
> > > after kernfs_get_active() we yield to let module removal trump calls to
> > > process a sysfs operation, while also preventing module removal if a sysfs
> > > operation is in already progress. This prevents the deadlock.
> > >
> > > This deadlock was first reported with the zram driver, however the live
> >
> > Looks not see the lock pattern you mentioned in zram driver, can you
> > share the related zram code?
>
> I recommend to not look at the zram driver, instead look at the
> test_sysfs driver as that abstracts the issue more clearly and uses

Looks test_sysfs isn't in linus tree, where can I find it? Also please
update your commit log about this wrong info if it can't be applied on
zram.

> two different locks as an example. The point is that if on module
> removal *any* lock is used which is *also* used on the sysfs file
> created by the module, you can deadlock.
>
> > > And this can lead to this condition:
> > >
> > > CPU A CPU B
> > > foo_store()
> > > foo_exit()
> > > mutex_lock(&foo)
> > > mutex_lock(&foo)
> > > del_gendisk(some_struct->disk);
> > > device_del()
> > > device_remove_groups()
> >
> > I guess the deadlock exists if foo_exit() is called anywhere. If yes,
> > look the issue may not be related with removing module directly, right?
>
> No, the reason this can deadlock is that the module exit routine will
> patiently wait for the sysfs / kernfs files to be stop being used,

Can you share the code which waits for the sysfs / kernfs files to be
stop being used? And why does it make a difference in case of being
called from module_exit()?

Thanks,
Ming