[PATCH v2 1/1] rcu/tree: Drop the lock before entering to page allocator

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

If the kernel is built with CONFIG_PROVE_RAW_LOCK_NESTING
option, the lockedp will complain about violation of the
nesting rules:

<snip>
[   28.060389] =============================
[   28.060389] [ BUG: Invalid wait context ]
[   28.060389] 5.8.0-rc3-rcu #211 Tainted: G            E
[   28.060389] -----------------------------
[   28.060390] vmalloc_test/0/523 is trying to lock:
[   28.060390] ffff96df7ffe0228 (&zone->lock){-.-.}-{3:3}, at: get_page_from_freelist+0xcf0/0x16d0
[   28.060391] other info that might help us debug this:
[   28.060391] context-{5:5}
[   28.060392] 2 locks held by vmalloc_test/0/523:
[   28.060392]  #0: ffffffffc06750d0 (prepare_for_test_rwsem){++++}-{4:4}, at: test_func+0x76/0x240 [test_vmalloc]
[   28.060393]  #1: ffff96df5fa1d390 (krc.lock){..-.}-{2:2}, at: kvfree_call_rcu+0x5c/0x230
[   28.060395] stack backtrace:
[   28.060395] CPU: 0 PID: 523 Comm: vmalloc_test/0 Tainted: G            E     5.8.0-rc3-rcu #211
[   28.060395] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[   28.060396] Call Trace:
[   28.060397]  dump_stack+0x96/0xd0
[   28.060397]  __lock_acquire.cold.65+0x166/0x2d7
[   28.060398]  ? find_held_lock+0x2d/0x90
[   28.060399]  lock_acquire+0xad/0x370
[   28.060400]  ? get_page_from_freelist+0xcf0/0x16d0
[   28.060401]  ? mark_held_locks+0x48/0x70
[   28.060402]  _raw_spin_lock+0x25/0x30
[   28.060403]  ? get_page_from_freelist+0xcf0/0x16d0
[   28.060404]  get_page_from_freelist+0xcf0/0x16d0
[   28.060405]  ? __lock_acquire+0x3ee/0x1b90
[   28.060407]  __alloc_pages_nodemask+0x16a/0x3a0
[   28.060408]  __get_free_pages+0xd/0x30
[   28.060409]  kvfree_call_rcu+0x18a/0x230
<snip>

Internally the kfree_rcu() uses raw_spinlock_t whereas the
page allocator internally deals with spinlock_t to access
to its zones.

In order to prevent such vialation that is in question we
can drop the internal raw_spinlock_t before entering to
the page allocaor.

Short changelog (v1 -> v2):
    - rework the commit message;
    - rework the patch making it smaller;
    - add more comments.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 21c2fa5bd8c3..2de112404121 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3287,6 +3287,8 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
 		return false;
 
 	lockdep_assert_held(&krcp->lock);
+	lockdep_assert_irqs_disabled();
+
 	idx = !!is_vmalloc_addr(ptr);
 
 	/* Check if a new block is required. */
@@ -3306,6 +3308,29 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
 			if (IS_ENABLED(CONFIG_PREEMPT_RT))
 				return false;
 
+			/*
+			 * If built with CONFIG_PROVE_RAW_LOCK_NESTING option,
+			 * the lockedp will complain about violation of the
+			 * nesting rules. It does the raw_spinlock vs. spinlock
+			 * nesting checks.
+			 *
+			 * That is why we drop the raw lock. Please note IRQs are
+			 * still disabled it guarantees that the "current" stays
+			 * on the same CPU later on when the raw lock is taken
+			 * back.
+			 *
+			 * It is important because if the page allocator is invoked
+			 * in fully preemptible context, it can be that we get a page
+			 * but end up on another CPU. That another CPU might not need
+			 * a page because of having some extra spots in its internal
+			 * array for pointer collecting. Staying on same CPU eliminates
+			 * described issue.
+			 *
+			 * Dropping the lock also reduces the critical section by
+			 * the time taken by the page allocator to obtain a page.
+			 */
+			raw_spin_unlock(&krcp->lock);
+
 			/*
 			 * NOTE: For one argument of kvfree_rcu() we can
 			 * drop the lock and get the page in sleepable
@@ -3315,6 +3340,8 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
 			 */
 			bnode = (struct kvfree_rcu_bulk_data *)
 				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
+
+			raw_spin_lock(&krcp->lock);
 		}
 
 		/* Switch to emergency path. */
-- 
2.20.1