From: Matthew Wilcox <willy@infradead.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>,
Alexey Dobriyan <adobriyan@gmail.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
linux-security-module@vger.kernel.org,
Paul Moore <paul@paul-moore.com>,
Stephen Smalley <stephen.smalley.work@gmail.com>,
Eric Paris <eparis@parisplace.org>,
selinux@vger.kernel.org, Casey Schaufler <casey@schaufler-ca.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU
Date: Sun, 13 Dec 2020 16:32:34 +0000 [thread overview]
Message-ID: <20201213163234.GH2443@casper.infradead.org> (raw)
In-Reply-To: <87tusplqwf.fsf@x220.int.ebiederm.org>
On Sun, Dec 13, 2020 at 08:30:40AM -0600, Eric W. Biederman wrote:
> Stephen Brennan <stephen.s.brennan@oracle.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.
next prev parent reply other threads:[~2020-12-13 16:34 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-04 0:02 [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU Stephen Brennan
2020-12-12 20:55 ` Matthew Wilcox
2020-12-13 14:22 ` Eric W. Biederman
2020-12-13 16:29 ` Matthew Wilcox
2020-12-13 23:00 ` Paul Moore
2020-12-15 18:09 ` Casey Schaufler
2020-12-15 22:04 ` Eric W. Biederman
2020-12-15 22:53 ` Casey Schaufler
2020-12-16 1:05 ` Stephen Brennan
2020-12-14 18:45 ` Casey Schaufler
2020-12-14 18:15 ` Stephen Brennan
2020-12-13 14:30 ` Eric W. Biederman
2020-12-13 16:32 ` Matthew Wilcox [this message]
2020-12-14 17:19 ` Stephen Brennan
2020-12-15 21:45 ` Eric W. Biederman
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