Messages in this thread | | | Date | Tue, 20 Aug 2019 11:12:10 +1000 | From | Dave Chinner <> | Subject | Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-) |
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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@fromorbit.com
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