block: nr_sects_write(): Disable preemption on seqcount write

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: c83f6bf ("block: add partition resize function to blkpg ioctl") Cc: <[email protected]> Signed-off-by: Ahmed S. Darwish <[email protected]> Reviewed-by: Sebastian Andrzej Siewior <[email protected]> Signed-off-by: Jens Axboe <[email protected]>
mrcsosa · Jun 5, 2020 · 15b81ce · 15b81ce
1 parent d24de76
commit 15b81ce
Showing 1 changed file with 2 additions and 0 deletions.
diff --git a/block/blk.h b/block/blk.h
@@ -420,9 +420,11 @@ 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;