On 2013年01月11日 06:38, Andrew Morton wrote:
On Wed, 9 Jan 2013 11:34:19 +0800
Fan Du<fan.du@xxxxxxxxxxxxx> wrote:
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...
Ouch.
And even if the preemping task is preemptible, it will spend an entire
timeslice pointlessly spinning, which isn't very good.
So disable preempt when acquiring every i_size_seqcount *write* lock will
cure the problem.
...
--- 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();
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;
afacit all write_seqcount_begin()/read_seqretry() sites are vulnerable
to this problem. Would it not be better to do the preempt_disable() in
write_seqcount_begin()?
IMHO, write_seqcount_begin/write_seqcount_end are often wrapped by mutex,
this gives higher priority task a chance to sleep, and then lower priority task
get cpu to unlock, so avoid the problematic scenario this patch describing.
But in i_size_write case, I could only find disable preempt a good choice before
someone else has better idea :)
Possible problems:
- mm/filemap_xip.c does disk I/O under write_seqcount_begin().
- dev_change_name() does GFP_KERNEL allocations under write_seqcount_begin()
- I didn't review u64_stats_update_begin() callers.
But I think calling schedule() under preempt_disable() is OK anyway?
--
浮沉随浪只记今朝笑
--fan
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