linux-kernel - Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP

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Date:   Sun, 13 Dec 2020 16:32:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

On Sun, Dec 13, 2020 at 08:30:40AM -0600, Eric W. Biederman wrote:
> Stephen Brennan <stephen.s.brennan@...cle.com> writes:
> 
> > The pid_revalidate() function requires dropping from RCU into REF lookup
> > mode. When many threads are resolving paths within /proc in parallel,
> > this can result in heavy spinlock contention as each thread tries to
> > grab a reference to the /proc dentry lock (and drop it shortly
> > thereafter).
> 
> I am feeling dense at the moment.  Which lock specifically are you
> referring to?  The only locks I can thinking of are sleeping locks,
> not spinlocks.

Stephen may have a better answer than this, but our mutex implementation
spins if the owner is still running, so he may have misspoken slightly.
He's testing on a giant system with hundreds of CPUs, so a mutex is
going to behave like a spinlock for him.

> Why do we need to test flags here at all?
> Why can't the code simply take an rcu_read_lock unconditionally and just
> pass flags into do_pid_update_inode?

Hah!  I was thinking about that possibility this morning, and I was
going to ask you that question.