toast operations while locking a buffer

Hi all

I see heap_tuple_toast_attrs() calls in heapam.c.
When they are called from heap_update/delete()
,a buffer is locked.
OTOH heap_tuple_toast_attrs() calls heap_open(..,
.., RowExclusiveLock). Hmm,try to acquire a
RowExclusiveLock while locking a buffer.
Is there no problem ?

Regards.

Hiroshi Inoue