srcu: Push srcu_node allocation to GP when non-preemptible

When the srcutree.convert_to_big and srcutree.big_cpu_lim kernel boot
parameters specify initialization-time allocation of the srcu_node
tree for statically allocated srcu_struct structures (for example, in
DEFINE_SRCU() at build time instead of init_srcu_struct() at runtime),
init_srcu_struct_nodes() will attempt to dynamically allocate this tree
at the first run-time update-side use of this srcu_struct structure,
but while holding a raw spinlock.  Because the memory allocator can
acquire non-raw spinlocks, this can result in lockdep splats.

This commit therefore uses the same SRCU_SIZE_ALLOC trick that is used
when the first run-time update-side use of this srcu_struct structure
happens before srcu_init() is called.  The actual allocation then takes
place from workqueue context at the ends of upcoming SRCU grace periods.

[boqun: Adjust the sha1 of the Fixes tag]

Fixes: 175b45e ("srcu: Use raw spinlocks so call_srcu() can be used under preempt_disable()")
Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>

Author: paulmckrcu

kernel/rcu/srcutree.c

@@ -227,9 +227,12 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	ssp->srcu_sup->srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL;
 	ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
-		if (!init_srcu_struct_nodes(ssp, is_static ? GFP_ATOMIC : GFP_KERNEL))
+		if (!preemptible())
+			WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_ALLOC);
+		else if (init_srcu_struct_nodes(ssp, GFP_KERNEL))
+			WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_BIG);
+		else
 			goto err_free_sda;
-		WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_BIG);
 	}
 	ssp->srcu_sup->srcu_ssp = ssp;
 	smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed,