Messages in this thread | | | Date | Mon, 25 May 2020 11:56:13 +0200 | From | "Ahmed S. Darwish" <> | Subject | Re: [PATCH v1 04/25] block: nr_sects_write(): Disable preemption on seqcount write |
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Peter Zijlstra <peterz@infradead.org> wrote: > 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.
Correct.
I just felt though that this would be too much for a 'Cc: stable' patch.
In another (in-progress) seqlock.h patch series, all of the seqcount_t call sites that are used for 64-bit values tearing protection on 32-bit kernels are transformed to the u64_stats_sync.h API.
Thanks,
-- Ahmed S. Darwish Linutronix GmbH
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