linux-kernel - [PATCH 3.2 35/87] include/linux/fs.h: disable preempt when acquire i_size

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Message-ID: <lsq.1385387069.690243829@decadent.org.uk>
Date:	Mon, 25 Nov 2013 13:44:29 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Fan Du" <fan.du@...driver.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.2 35/87] include/linux/fs.h: disable preempt when
 acquire i_size_seqcount write lock

3.2.53-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Fan Du <fan.du@...driver.com>

commit 74e3d1e17b2e11d175970b85acd44f5927000ba2 upstream.

Two rt tasks bind to one CPU core.

The higher priority rt task A preempts a lower priority rt task B which
has already taken the write seq lock, and then the higher priority rt
task A try to acquire read seq lock, it's doomed to lockup.

rt task A with lower priority: call write
i_size_write                                        rt task B with higher priority: call sync, and preempt task A
  write_seqcount_begin(&inode->i_size_seqcount);    i_size_read
  inode->i_size = i_size;                             read_seqcount_begin <-- lockup here...

So disable preempt when acquiring every i_size_seqcount *write* lock will
cure the problem.

Signed-off-by: Fan Du <fan.du@...driver.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/fs.h | 2 ++
 1 file changed, 2 insertions(+)

--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -911,9 +911,11 @@ static inline loff_t i_size_read(const s
 static inline void i_size_write(struct inode *inode, loff_t i_size)
 {
 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	preempt_disable();
 	write_seqcount_begin(&inode->i_size_seqcount);
 	inode->i_size = i_size;
 	write_seqcount_end(&inode->i_size_seqcount);
+	preempt_enable();
 #elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPT)
 	preempt_disable();
 	inode->i_size = i_size;

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