[v2,5/6] block: nr_sects_write(): Disable preemption on seqcount write
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Message ID 20200603144949.1122421-6-a.darwish@linutronix.de
State New
Headers show
Series
  • seqlock: seqcount_t call sites bugfixes
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Commit Message

Ahmed S. Darwish June 3, 2020, 2:49 p.m. UTC
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(+)

Comments

Jens Axboe June 5, 2020, 3:22 a.m. UTC | #1
On 6/3/20 8:49 AM, 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.

Applied, thanks.

Patch
diff mbox series

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;