Re: Bounce buffer deadlock

[Steve Lord <lord@sgi.com>]

> 
> 
> On Fri, 29 Jun 2001, Steve Lord wrote:
> 
> > 
> > Has anyone else seen a hang like this:
> > 
> >   bdflush()
> >     flush_dirty_buffers()
> >       ll_rw_block()
> > 	submit_bh(buffer X)
> > 	  generic_make_request()
> > 	    __make_request()
> > 	        create_bounce()
> > 		  alloc_bounce_page()
> > 		    alloc_page()
> > 		      try_to_free_pages()
> > 			do_try_to_free_pages()
> > 			  page_launder()
> > 			    try_to_free_buffers( , 2)  -- i.e. wait for buffers
> > 			      sync_page_buffers()
> > 				__wait_on_buffer(buffer X)
> > 
> > Where the buffer head X going in the top of the stack is the same as the on
> e
> > we wait on at the bottom.
> > 
> > There still seems to be nothing to prevent the try to free buffers from
> > blocking on a buffer like this. Setting a flag on the buffer around the
> > create_bounce call, and skipping it in the try_to_free_buffers path would
> > be one approach to avoiding this.
> 
> Yes there is a bug: Linus is going to put a new fix soon. 
> 
> > I hit this in 2.4.6-pre6, and I don't see anything in the ac series to prot
> ect
> > against it.
> 
> Thats because the -ac series does not contain the __GFP_BUFFER/__GFP_IO
> moditications which are in the -ac series.

It looks to me as if all memory allocations of type GFP_BUFFER which happen
in generic_make_request downwards can hit the same type of deadlock, so
bounce buffers, the request functions of the raid and lvm paths can all
end up in try_to_free_buffers on a buffer they themselves hold the lock on.

If the fix is to avoid page_launder in these cases then the number of
occurrences when an alloc_pages fails will go up. I was attempting to
come up with a way of making try_to_free_buffers fail on buffers which
are being processed in the generic_make_request path by marking them,
the problem is there is no single place to reset the state of a buffer so
that  try_to_free_buffers will wait for it. Doing it after the end of the
loop in generic_make_request is race prone to say the least.

Steve