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flex_proportions: make fprop_new_period() hardirq safe

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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: a91befde35 ("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>
This commit is contained in:
Jan Kara 2026-01-21 12:27:30 +01:00 • committed by Andrew Morton
parent d54887e8e1
commit dd9e2f5b38
1 changed files with 3 additions and 2 deletions
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5
lib/flex_proportions.c
View file

@ -64,13 +64,14 @@ void fprop_global_destroy(struct fprop_global *p)
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_global *p, int periods)
percpu_counter_add(&p->events, -events);
p->period += periods;
write_seqcount_end(&p->sequence);
preempt_enable_nested();
local_irq_restore(flags);
return true;
}