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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.7 348/376] block: nr_sects_write(): Disable preemption on seqcount write
Date:   Fri, 19 Jun 2020 16:34:26 +0200
Message-Id: <20200619141726.799900818@linuxfoundation.org>
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From: Ahmed S. Darwish <a.darwish@linutronix.de>

commit 15b81ce5abdc4b502aa31dff2d415b79d2349d2f upstream.

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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 block/blk.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/block/blk.h
+++ b/block/blk.h
@@ -470,9 +470,11 @@ static inline sector_t part_nr_sects_rea
 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;