Re: [PATCH v1 04/25] block: nr_sects_write(): Disable preemption on seqcount write

Peter Zijlstra
2020-05-22T16:39:21Z

On Tue, May 19, 2020 at 11:45:26PM +0200, Ahmed S. Darwish wrote:
> For optimized block readers not holding a mutex, the "number of sectors"
> 64-bit value is protected from tearing on 32-bit architectures by a
> sequence counter.
>
> Disable preemption before entering that sequence counter's write side
> critical section. Otherwise, the read side can preempt the write side
> section and spin for the entire scheduler tick. If the reader belongs to
> a real-time scheduling class, it can spin forever and the kernel will
> livelock.
>
> Fixes: c83f6bf98dc1 ("block: add partition resize function to blkpg ioctl")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> block/blk.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/block/blk.h b/block/blk.h
> index 0a94ec68af32..151f86932547 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -470,9 +470,11 @@ static inline sector_t part_nr_sects_read(struct hd_struct *part)
> static inline void part_nr_sects_write(struct hd_struct *part, sector_t size)
> {
> #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
> + preempt_disable();
> write_seqcount_begin(&part->nr_sects_seq);
> part->nr_sects = size;
> write_seqcount_end(&part->nr_sects_seq);
> + preempt_enable();
> #elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPTION)
> preempt_disable();
> part->nr_sects = size;

This does look like something that include/linux/u64_stats_sync.h could
help with.