linux-kernel - [PATCH 2/2] rcu/tree: use __GFP_NO

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Date:   Sun,  9 Aug 2020 22:43:54 +0200
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: [PATCH 2/2] rcu/tree: use __GFP_NO_LOCKS flag

Enter the page allocator with newly introduced __GFP_NO_LOCKS flag
instead of former GFP_NOWAIT | __GFP_NOWARN sequence. Such approach
address two concerns. See them below:

a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep complains
about violation("BUG: Invalid wait context") of the nesting rules. It
does the raw_spinlock vs. spinlock nesting checks, i.e. it is not legal
to acquire a spinlock_t while holding a raw_spinlock_t.

Internally the kfree_rcu() uses raw_spinlock_t whereas the page allocator
internally deals with spinlock_t to access to its zones. The code also
can be broken from higher level of view:
<snip>
    raw_spin_lock(&some_lock);
    kfree_rcu(some_pointer, some_field_offset);
<snip>

b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t
is converted into sleepable variant. Invoking the page allocator from
atomic contexts leads to: "BUG: scheduling while atomic".

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 30e7e252b9e7..48cb64800108 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3327,7 +3327,7 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
 			 * pages are available.
 			 */
 			bnode = (struct kvfree_rcu_bulk_data *)
-				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
+				__get_free_page(__GFP_NO_LOCKS);
 		}
 
 		/* Switch to emergency path. */
-- 
2.20.1