Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI

[Jan Kara]

On Fri 09-10-15 11:03:30, Nikolay Borisov wrote:
> On 10/09/2015 10:37 AM, Hillf Danton wrote:
> >>>> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio)
> >>>>  			if (bio->bi_error)
> >>>>  				buffer_io_error(bh);
> >>>>  		} while ((bh = bh->b_this_page) != head);
> >>>> -		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> >>>>  		local_irq_restore(flags);
> >>>
> >>> What if it takes 100ms to unlock after IRQ restored?
> >>
> >> I'm not sure I understand in what direction you are going? Care to
> >> elaborate?
> >>
> > Your change introduces extra time cost the lock waiter has to pay in
> > the case that irq happens before the lock is released.
> 
> [CC filesystem and mm people. For reference the thread starts here:
>  http://thread.gmane.org/gmane.linux.kernel/2056996 ]
> 
> Right, I see what you mean and it's a good point but when doing the
> patches I was striving for correctness and starting a discussion, hence
> the RFC. In any case I'd personally choose correctness over performance
> always ;).
> 
> As I'm not an fs/ext4 expert and have added the relevant parties (please
> use reply-all from now on so that the thread is not being cut in the
> middle) who will be able to say whether it impact is going to be that
> big. I guess in this particular code path worrying about this is prudent
> as writeback sounds like a heavily used path.
> 
> Maybe the problem should be approached from a different angle e.g.
> drain_all_pages and its reliance on the fact that the IPI will always be
> delivered in some finite amount of time? But what if a cpu with disabled
> interrupts is waiting on the task issuing the IPI?

So I have looked through your patch and also original report (thread starts
here: https://lkml.org/lkml/2015/10/8/341) and IMHO one question hasn't
been properly answered yet: Who is holding BH_Uptodate_Lock we are spinning
on? You have suggested in https://lkml.org/lkml/2015/10/8/464 that it was
__block_write_full_page_endio() call but that cannot really be the case.
BH_Uptodate_Lock is used only in IO completion handlers -
end_buffer_async_read, end_buffer_async_write, ext4_finish_bio. So there
really should be some end_io function running on some other CPU which holds
BH_Uptodate_Lock for that buffer.

BTW: I suppose the filesystem uses 4k blocksize, doesn't it?

								Honza

> >>>> +		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> >>>>  		if (!under_io) {
> >>>>  #ifdef CONFIG_EXT4_FS_ENCRYPTION
> >>>>  			if (ctx)
> >>>> --
> >>>> 2.5.0
> >>>
> > 
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-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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