How oracle locks child table with foreign keys?

Question

I have 3 tables. tab1 , tab2,tab3.
tab2 has FK on tab1.
tab3 has FK on tab2.All the FKs are indexed.
One transaction in my code changes tab3.
One transaction in my code changes tab1,tab2 and tab3.
I am facing deadlock issues on tab3 when both transactions work concurrently.
If I lock tab1 with "for update nowait " at the start of both transaction,will the problem be solved?

Answer 1

Let me elaborate on my problem
TAB1          
=====          
a1 b1 c1     
a2 b2 c2     
a3 b3 c3     
TAB2               
=====               
x1 y1 z1 a1          
x2 y2 z2 a2          
x3 y3 z3 a3          
(FK indexed)          
TAB3
=====
p1 q1 r1 y1
p2 q2 r2 y2
p3 q3 r3 y3
(FK indexed)
Transaction 1 for session1
select * from TAB1 where col1 = a1 for update nowait;
doing activities on tab2 and tab3 depending on the foreign key joins in loop.
Transaction 2 for session 2
select * from TAB1 where col1 = a2 for update nowait;
doing activities on tab2 and tab3 depending on the foreign key joins in loop.
1) Please let me know whether these sessions can head towards deadlock.
2) If some session try to update TAB3 row of y1 when session 1 is still working , how will system behave?
will it go for blocked session or go for dead lock or get an exception message?
Edited by: user9974355 on Dec 17, 2008 11:58 PM
Edited by: user9974355 on Dec 18, 2008 12:00 AM