linux-kernel - Re: [PATCH v4 1/4] mm: reduce the rcu lock duration

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YpYSjw/T8jTGr7e8@casper.infradead.org>
Date:   Tue, 31 May 2022 14:05:19 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        peterx@...hat.com, apopple@...dia.com, ying.huang@...el.com,
        osalvador@...e.de, mike.kravetz@...cle.com,
        songmuchun@...edance.com, hch@....de, dhowells@...hat.com,
        cl@...ux.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/4] mm: reduce the rcu lock duration

On Tue, May 31, 2022 at 01:58:31PM +0100, Matthew Wilcox wrote:
> On Mon, May 30, 2022 at 07:30:13PM +0800, Miaohe Lin wrote:
> > Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
> > extends the period of the rcu_read_lock until after the permissions checks
> > are done to prevent the task pointed to from changing from under us. But
> > the task_struct refcount is also taken at that time, the reference to task
> > is guaranteed to be stable. So it's unnecessary to extend the period of
> > the rcu_read_lock. Release the rcu lock after task refcount is successfully
> > grabbed to reduce the rcu holding time.
> 
> But why bother?  You know the RCU read lock isn't a "real" lock, right?

Looking over this code some more, I think this may harm performance.
ptrace_may_access() itself takes the rcu_read_lock().  So we currently
have:

rcu_read_lock()
rcu_read_lock();
rcu_read_unlock();
rcu_read_unlock();

In at least one RCU configuration, rcu_read_lock() maps to
preempt_disable().  Nested preempt_disable() just bump a counter, while
that counter reaching zero incurs some actual work.  So nested
rcu_read_lock() can be better than sequential lock/unlock/lock/unlock.

This needs far better justification.