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

Linus Torvalds [off-list ref] writes:

On Wed, Feb 12, 2020 at 12:41 PM Al Viro [off-list ref] wrote:

quoted

On Wed, Feb 12, 2020 at 08:38:33PM +0000, Al Viro wrote:

quoted

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