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()?
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?
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