linux-ext4 - Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

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Message-ID: <20190820011210.GP7777@dread.disaster.area>
Date:   Tue, 20 Aug 2019 11:12:10 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jan Kara <jack@...e.cz>, Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.com>, linux-xfs@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

On Mon, Aug 19, 2019 at 09:38:41AM -0300, Jason Gunthorpe wrote:
> On Mon, Aug 19, 2019 at 07:24:09PM +1000, Dave Chinner wrote:
> 
> > So that leaves just the normal close() syscall exit case, where the
> > application has full control of the order in which resources are
> > released. We've already established that we can block in this
> > context.  Blocking in an interruptible state will allow fatal signal
> > delivery to wake us, and then we fall into the
> > fatal_signal_pending() case if we get a SIGKILL while blocking.
> 
> The major problem with RDMA is that it doesn't always wait on close() for the
> MR holding the page pins to be destoyed. This is done to avoid a
> deadlock of the form:
> 
>    uverbs_destroy_ufile_hw()
>       mutex_lock()
>        [..]
>         mmput()
>          exit_mmap()
>           remove_vma()
>            fput();
>             file_operations->release()

I think this is wrong, and I'm pretty sure it's an example of why
the final __fput() call is moved out of line.

		fput()
		  fput_many()
		    task_add_work(f, __fput())

and the call chain ends there.

Before the syscall returns to userspace, it then runs the __fput()
call through the task_work_run() interfaces, and hence the call
chain is just:

	task_work_run
	  __fput
>             file_operations->release()
>              ib_uverbs_close()
>               uverbs_destroy_ufile_hw()
>                mutex_lock()   <-- Deadlock

And there is no deadlock because nothing holds the mutex at this
point.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com