linux-kernel - [PATCH RT 08/12] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq

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Message-Id: <20231018194833.651674-9-joseph.salisbury@canonical.com>
Date:   Wed, 18 Oct 2023 15:48:29 -0400
From:   Joseph Salisbury <joseph.salisbury@...onical.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <daniel.wagner@...e.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Clark Williams <williams@...hat.com>,
        Pavel Machek <pavel@...x.de>,
        Joseph Salisbury <joseph.salisbury@...onical.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>
Subject: [PATCH RT 08/12] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save().

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

v5.15.133-rt70-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


__build_all_zonelists() acquires zonelist_update_seq by first disabling
interrupts via local_irq_save() and then acquiring the seqlock with
write_seqlock(). This is troublesome and leads to problems on
PREEMPT_RT. The problem is that the inner spinlock_t becomes a sleeping
lock on PREEMPT_RT and must not be acquired with disabled interrupts.

The API provides write_seqlock_irqsave() which does the right thing in
one step.
printk_deferred_enter() has to be invoked in non-migrate-able context to
ensure that deferred printing is enabled and disabled on the same CPU.
This is the case after zonelist_update_seq has been acquired.

There was discussion on the first submission that the order should be:
	local_irq_disable();
	printk_deferred_enter();
	write_seqlock();

to avoid pitfalls like having an unaccounted printk() coming from
write_seqlock_irqsave() before printk_deferred_enter() is invoked. The
only origin of such a printk() can be a lockdep splat because the
lockdep annotation happens after the sequence count is incremented.
This is exceptional and subject to change.

It was also pointed that PREEMPT_RT can be affected by the printk
problem since its write_seqlock_irqsave() does not really disable
interrupts. This isn't the case because PREEMPT_RT's printk
implementation differs from the mainline implementation in two important
aspects:
- Printing happens in a dedicated threads and not at during the
  invocation of printk().
- In emergency cases where synchronous printing is used, a different
  driver is used which does not use tty_port::lock.

Acquire zonelist_update_seq with write_seqlock_irqsave() and then defer
printk output.

Fixes: 1007843a91909 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock")
Acked-by: Michal Hocko <mhocko@...e.com>
Reviewed-by: David Hildenbrand <david@...hat.com>
Link: https://lore.kernel.org/r/20230623201517.yw286Knb@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
(cherry picked from commit 4d1139baae8bc4fff3728d1d204bdb04c13dbe10)
Signed-off-by: Joseph Salisbury <joseph.salisbury@...onical.com>
---
 mm/page_alloc.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 33355028122a..174bcc23d5fd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6419,19 +6419,17 @@ static void __build_all_zonelists(void *data)
 	unsigned long flags;
 
 	/*
-	 * Explicitly disable this CPU's interrupts before taking seqlock
-	 * to prevent any IRQ handler from calling into the page allocator
-	 * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
+	 * The zonelist_update_seq must be acquired with irqsave because the
+	 * reader can be invoked from IRQ with GFP_ATOMIC.
 	 */
-	local_irq_save(flags);
+	write_seqlock_irqsave(&zonelist_update_seq, flags);
 	/*
-	 * Explicitly disable this CPU's synchronous printk() before taking
-	 * seqlock to prevent any printk() from trying to hold port->lock, for
+	 * Also disable synchronous printk() to prevent any printk() from
+	 * trying to hold port->lock, for
 	 * tty_insert_flip_string_and_push_buffer() on other CPU might be
 	 * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
 	 */
 	printk_deferred_enter();
-	write_seqlock(&zonelist_update_seq);
 
 #ifdef CONFIG_NUMA
 	memset(node_load, 0, sizeof(node_load));
@@ -6464,9 +6462,8 @@ static void __build_all_zonelists(void *data)
 #endif
 	}
 
-	write_sequnlock(&zonelist_update_seq);
 	printk_deferred_exit();
-	local_irq_restore(flags);
+	write_sequnlock_irqrestore(&zonelist_update_seq, flags);
 }
 
 static noinline void __init
-- 
2.34.1