Recursion in non-recursive mutex when using the grant table free callbacks

[Pratyush Yadav]

Hi,

Currently, the grant table free callbacks can not work. This is
because of a recursion on a non-recursive mutex that causes a kernel
panic. The cause of the recursion is: check_free_callbacks() is always
called with the lock gnttab_list_lock held. So, the callback function
is called with the lock held. So, when the client uses any of the
grant reference allocation methods get_free_entries() is called, which
tries to acquire gnttab_list_lock(grant_table.c:77 [0]), causing a
recursion on the lock.

I'm not sure what the correct fix would be though. One way I can think
of is that check_free_callback() should be called without the lock
held. But with this fix, it is possible for the callback to be called
even though the grant references it needs are not available. This
would happen when another thread takes those references while the
current thread has completed the check if(gnttab_free_count >=
callback->count) but has not yet called the callback
(grant_table,c:105 [1]).

I think a better way to fix this would be to have a check in
get_free_entries() whether the current thread holds the lock, so it
does not try to acquire the lock if the current thread already holds
it.

If you agree, I will send a patch.



[0] https://github.com/freebsd/freebsd/blob/master/sys/dev/xen/grant_table/grant_table.c#L77
[1] https://github.com/freebsd/freebsd/blob/master/sys/dev/xen/grant_table/grant_table.c#L105

-- 
Regards,
Pratyush Yadav