Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, Feb 12, 2020 at 12:41 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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.

Eric