[PATCH] Disable preempt when acquire i_size_seqcount write lock

From: Fan Du
Date: Fri Dec 21 2012 - 03:41:11 EST

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@xxxxxxxxxxxxx>
include/linux/fs.h | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index db84f77..1b69e87 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -758,9 +758,11 @@ static inline loff_t i_size_read(const struct inode *inode)
static inline void i_size_write(struct inode *inode, loff_t i_size)
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+ preempt_disable();
inode->i_size = i_size;
+ preempt_enable();
#elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPT)
inode->i_size = i_size;

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