Re: [patch] __block_write_full_page bug

[Nick Piggin]

On Tue, 2005-04-26 at 04:50 -0700, Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> >
> >  When running
> >  	fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
> >  on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
> >  page size over loopback to an image file on a tmpfs filesystem, I would
> >  very quickly hit
> >  	BUG_ON(!buffer_async_write(bh));
> >  in fs/buffer.c:end_buffer_async_write
> > 
> >  It seems that more than one request would be submitted for a given bh
> >  at a time. __block_write_full_page looks like the culprit - with the
> >  following patch things are very stable.
> 
> What's the bug?  I don't see it.
> 

Ah, the bug is that end_buffer_async_write first does
	BUG_ON(!buffer_async_write(bh));
then a bit later does
	clear_buffer_async_write(bh);

That's where it was blowing up for me, because end_buffer_async_write
was being run twice for that buffer.

Or did you mean *how* is it being run twice? I didn't exactly find
the stack traces involved, but I imagine that simply testing
buffer_async_write catches other requests in flight - ie. we've
lost track of exactly which ones we own.


> Was an ENOSPC involved?
> 

No.

> Those tests for buffer_async_write(bh) are redundant now, aren't they?

They are, yes. Sorry I noticed that earlier too - they should probably
be BUG_ON(!buffer_async_write(bh)) instead. Would make things clearer.

Nick



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