Received-SPF: pass (google.com: best guess record for domain of linux-ext4-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 21 Aug 2019 16:48:10 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
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
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)
Message-ID: <20190821194810.GI8653@ziepe.ca>
References: <20190816190528.GB371@iweiny-DESK2.sc.intel.com>
 <20190817022603.GW6129@dread.disaster.area>
 <20190819063412.GA20455@quack2.suse.cz>
 <20190819092409.GM7777@dread.disaster.area>
 <20190819123841.GC5058@ziepe.ca>
 <20190820011210.GP7777@dread.disaster.area>
 <20190820115515.GA29246@ziepe.ca>
 <20190821180200.GA5965@iweiny-DESK2.sc.intel.com>
 <20190821181343.GH8653@ziepe.ca>
 <20190821185703.GB5965@iweiny-DESK2.sc.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190821185703.GB5965@iweiny-DESK2.sc.intel.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-ext4-owner@vger.kernel.org
Precedence: bulk

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