__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.
Link: https://lkml.kernel.org/r/20230623201517.yw286Knb@linutronix.de
Fixes: 1007843a91 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
a2ebb51575