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Date: Wed, 21 Aug 2019 16:48:10 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)
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To: Ira Weiny <ira.weiny@intel.com>
Cc: Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, Dan Williams <dan.j.williams@intel.com>, Matthew Wilcox <willy@infradead.org>, Theodore Ts'o <tytso@mit.edu>, John Hubbard <jhubbard@nvidia.com>, Michal Hocko <mhocko@suse.com>, linux-xfs@vger.kernel.org, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org
List-ID: <linux-nvdimm@lists.01.org>

On Wed, Aug 21, 2019 at 11:57:03AM -0700, Ira Weiny wrote:

> > Oh, I didn't think we were talking about that. Hanging the close of
> > the datafile fd contingent on some other FD's closure is a recipe for
> > deadlock..
> 
> The discussion between Jan and Dave was concerning what happens when a user
> calls
> 
> fd = open()
> fnctl(...getlease...)
> addr = mmap(fd...)
> ib_reg_mr() <pin>
> munmap(addr...)
> close(fd)

I don't see how blocking close(fd) could work. Write it like this:

 fd = open()
 uverbs = open(/dev/uverbs)
 fnctl(...getlease...)
 addr = mmap(fd...)
 ib_reg_mr() <pin>
 munmap(addr...)
  <sigkill>

The order FD's are closed during sigkill is not deterministic, so when
all the fputs happen during a kill'd exit we could end up blocking in
close(fd) as close(uverbs) will come after in the close
list. close(uverbs) is the thing that does the dereg_mr and releases
the pin.

We don't need complexity with dup to create problems.

Jason