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

[Ira Weiny]

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()
>              ib_uverbs_close()
>               uverbs_destroy_ufile_hw()
>                mutex_lock()   <-- Deadlock
> 
> But, as I said to Ira earlier, I wonder if this is now impossible on
> modern kernels and we can switch to making the whole thing
> synchronous. That would resolve RDMA's main problem with this.

I'm still looking into this...  but my bigger concern is that the RDMA FD can
be passed to other processes via SCM_RIGHTS.  Which means the process holding
the pin may _not_ be the one with the open file and layout lease...

Ira