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SubjectRe: [PATCH] fs: Disable preempt when acquire i_size_seqcount write lock


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@windriver.com> 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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