From 40b3183d5a1c19369c3c15b9bce144f2ccf330f0 Mon Sep 17 00:00:00 2001
From: Fan Du <fan.du@windriver.com>
Date: Tue, 30 Apr 2013 15:27:27 -0700
Subject: [PATCH] include/linux/fs.h: disable preempt when acquire
 i_size_seqcount write lock

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@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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