+ flex_proportions-make-fprop_new_period-hardirq-safe.patch added to mm-hotfixes-unstable branch

+ flex_proportions-make-fprop_new_period-hardirq-safe.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: flex_proportions: make fprop_new_period() hardirq safe
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     flex_proportions-make-fprop_new_period-hardirq-safe.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/flex_proportions-make-fprop_new_period-hardirq-safe.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Jan Kara <jack@suse.cz>
Subject: flex_proportions: make fprop_new_period() hardirq safe
Date: Wed, 21 Jan 2026 12:27:30 +0100

Bernd has reported a lockdep splat from flexible proportions code that is
essentially complaining about the following race:

<timer fires>
run_timer_softirq - we are in softirq context
  call_timer_fn
    writeout_period
      fprop_new_period
        write_seqcount_begin(&p->sequence);

        <hardirq is raised>
        ...
        blk_mq_end_request()
	  blk_update_request()
	    ext4_end_bio()
	      folio_end_writeback()
		__wb_writeout_add()
		  __fprop_add_percpu_max()
		    if (unlikely(max_frac < FPROP_FRAC_BASE)) {
		      fprop_fraction_percpu()
			seq = read_seqcount_begin(&p->sequence);
			  - sees odd sequence so loops indefinitely

Note that a deadlock like this is only possible if the bdi has configured
maximum fraction of writeout throughput which is very rare in general but
frequent for example for FUSE bdis.  To fix this problem we have to make
sure write section of the sequence counter is irqsafe.

Link: https://lkml.kernel.org/r/20260121112729.24463-2-jack@suse.cz
Fixes: a91befde3503 ("lib/flex_proportions.c: remove local_irq_ops in fprop_new_period()")
Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Bernd Schubert <bernd@bsbernd.com>
Link: https://lore.kernel.org/all/9b845a47-9aee-43dd-99bc-1a82bea00442@bsbernd.com/
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/flex_proportions.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/lib/flex_proportions.c~flex_proportions-make-fprop_new_period-hardirq-safe
+++ a/lib/flex_proportions.c
@@ -64,13 +64,14 @@ void fprop_global_destroy(struct fprop_g
 bool fprop_new_period(struct fprop_global *p, int periods)
 {
 	s64 events = percpu_counter_sum(&p->events);
+	unsigned long flags;
 
 	/*
 	 * Don't do anything if there are no events.
 	 */
 	if (events <= 1)
 		return false;
-	preempt_disable_nested();
+	local_irq_save(flags);
 	write_seqcount_begin(&p->sequence);
 	if (periods < 64)
 		events -= events >> periods;
@@ -78,7 +79,7 @@ bool fprop_new_period(struct fprop_globa
 	percpu_counter_add(&p->events, -events);
 	p->period += periods;
 	write_seqcount_end(&p->sequence);
-	preempt_enable_nested();
+	local_irq_restore(flags);
 
 	return true;
 }
_

Patches currently in -mm which might be from jack@suse.cz are

flex_proportions-make-fprop_new_period-hardirq-safe.patch