Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Wed, 12 Feb 2020 16:48:14 -0800
From: Linus Torvalds <>
To: "Eric W. Biederman" <>
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

On Wed, Feb 12, 2020 at 1:48 PM Eric W. Biederman <> wrote:
> 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.

Yeah, that at least avoids some of the nasty locking while dying debug problems.

But the one I was more worried about was actually the lock contention
issue with lots of processes. The lock is basically a single global
lock in many situations - yes, it's technically per-ns, but in a lot
of cases you really only have one namespace anyway.

And we've had problems with global locks in this area before, notably
the one you call out:

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

Yeah, so it's not introducing a new issue, but it is potentially
making something we already know is bad even worse.

> 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.

That's what the original patch actually was, and I was hoping we could
avoid that thing.

An rwsem would be possibly better, since most cases by far are likely
about reading.

And yes, I'm very aware of the task_list_lock, but it's literally why
I don't want to make a new one.

I'm _hoping_ we can some day come up with something better than task_list_lock.


Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.