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Date: Wed, 12 Feb 2020 15:46:29 -0600
From: (Eric W. Biederman)
To: Linus Torvalds <>
Cc: Al Viro <>,  LKML <>,  Kernel Hardening <>,  Linux API <>,  Linux FS Devel <>,  Linux Security Module <>,  Akinobu Mita <>,  Alexey Dobriyan <>,  Andrew Morton <>,  Andy Lutomirski <>,  Daniel Micay <>,  Djalal Harouni <>,  "Dmitry V . Levin" <>,  Greg Kroah-Hartman <>,  Ingo Molnar <>,  "J . Bruce Fields" <>,  Jeff Layton <>,  Jonathan Corbet <>,  Kees Cook <>,  Oleg Nesterov <>,  Solar Designer <>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

Linus Torvalds <> writes:

> On Wed, Feb 12, 2020 at 12:41 PM Al Viro <> wrote:
>> On Wed, Feb 12, 2020 at 08:38:33PM +0000, Al Viro wrote:
>> >
>> > Wait, I thought the whole point of that had been to allow multiple
>> > procfs instances for the same userns?  Confused...
>> s/userns/pidns/, sorry
> Right, but we still hold the ref to it here...
> [ Looks more ]
> Oooh. No we don't. Exactly because we don't hold the lock, only the
> rcu lifetime, the ref can go away from under us. I see what your
> concern is.
> Ouch, this is more painful than I expected - the code flow looked so
> simple. I really wanted to avoid a new lock during process shutdown,
> because that has always been somewhat painful.

The good news is proc_flush_task isn't exactly called from process exit.
proc_flush_task is called during zombie clean up. AKA release_task.

So proc_flush_task isn't called with any locks held, and it is
called in a context where it can sleep.

Further after proc_flush_task does it's thing the code goes
and does "write_lock_irq(&task_list_lock);"

So the code is definitely serialized to one processor already.

What would be downside of having a mutex for a list of proc superblocks?
A mutex that is taken for both reading and writing the list.


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