On 11/9/23 5:45 PM, Paul E. McKenney wrote:
+void bpf_map_of_map_fd_put_ptr(void *ptr, bool need_defer)
+{
+ struct bpf_inner_map_element *element = ptr;
+
+ /* Do bpf_map_put() after a RCU grace period and a tasks trace
+ * RCU grace period, so it is certain that the bpf program which is
+ * manipulating the map now has exited when bpf_map_put() is
called.
+ */
+ if (need_defer)
"need_defer" should only happen from the syscall cmd? Instead of
adding rcu_head to each element, how about
"synchronize_rcu_mult(call_rcu, call_rcu_tasks)" here?
No. I have tried the method before, but it didn't work due to dead-lock
(will mention that in commit message in v2). The reason is that bpf
syscall program may also do map update through sys_bpf helper. Because
bpf syscall program is running with sleep-able context and has
rcu_read_lock_trace being held, so call synchronize_rcu_mult(call_rcu,
call_rcu_tasks) will lead to dead-lock.
Need to think of a less intrusive solution instead of adding rcu_head to each
element and lookup also needs an extra de-referencing.
May be the bpf_map_{update,delete}_elem(&outer_map, ....) should not be done by
the syscall program? Which selftest does it?
Can the inner_map learn that it has been deleted from an outer map that is used
in a sleepable prog->aux->used_maps? The bpf_map_free_deferred() will then wait
for a task_trace gp?
