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From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	"Sebastian A. Siewior" <bigeasy@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Jens Axboe <axboe@kernel.dk>, Vivek Goyal <vgoyal@redhat.com>,
	linux-block@vger.kernel.org
Subject: [PATCH v2 5/6] block: nr_sects_write(): Disable preemption on seqcount write
Date: Wed,  3 Jun 2020 16:49:48 +0200
Message-ID: <20200603144949.1122421-6-a.darwish@linutronix.de> (raw)
In-Reply-To: <20200603144949.1122421-1-a.darwish@linutronix.de>

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;
-- 
2.20.1


  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-03 14:49 [PATCH v2 0/6] seqlock: seqcount_t call sites bugfixes Ahmed S. Darwish
2020-06-03 14:49 ` [PATCH v2 1/6] net: core: device_rename: Use rwsem instead of a seqcount Ahmed S. Darwish
2020-06-03 14:49 ` [PATCH v2 2/6] net: phy: fixed_phy: Remove unused seqcount Ahmed S. Darwish
2020-06-03 16:19   ` Andrew Lunn
2020-06-03 14:49 ` [PATCH v2 3/6] u64_stats: Document writer non-preemptibility requirement Ahmed S. Darwish
2020-06-03 14:49 ` [PATCH v2 4/6] net: mdiobus: Disable preemption upon u64_stats update Ahmed S. Darwish
2020-06-03 14:49 ` Ahmed S. Darwish [this message]
2020-06-05  3:22   ` [PATCH v2 5/6] block: nr_sects_write(): Disable preemption on seqcount write Jens Axboe
2020-06-03 14:49 ` [PATCH v2 6/6] dma-buf: Remove custom seqcount lockdep class key Ahmed S. Darwish
2020-06-04  8:49   ` Daniel Vetter
2020-06-04  7:28 ` [PATCH v2 0/6] seqlock: seqcount_t call sites bugfixes Daniel Vetter
2020-06-04  8:41   ` Ahmed S. Darwish
2020-06-04 22:50 ` David Miller

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