linux-kernel - Re: [PATCH] lglock: make lg_lock_global() actually lock globally

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: <20100825141644.715258cc@bike.lwn.net>
Date:	Wed, 25 Aug 2010 14:16:44 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lglock: make lg_lock_global() actually lock globally

On Wed, 25 Aug 2010 13:00:59 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Grrr. Same disease as Nick and others. Why do you repeat the subject
> line in the body? Don't do that. We don't want the summary line twice
> in the commit message, and we don't want it twice in the email.
> 
> We simply don't want it twice. Full stop.

Sorry, I just pasted in the "git format-patch" output.  Will never ever
ever do it again I promise cross my heart.

> > lg_lock_global() currently only acquires spinlocks for online CPUs, but
> > it's meant to lock all possible CPUs.  At Nick's suggestion, change
> > for_each_online_cpu() to for_each_possible_cpu() to get the expected
> > behavior.  
> 
> Can you say what this actually matters for? Don't we do stop-machine
> for CPU hotplug anyway? And if we don't, shouldn't we? Exactly because
> otherwise "for_each_online_cpu()" is always racy (and that has nothing
> to do with the lglock).

As I understand it from Nick (after I asked him why the two lock
primitives were identical): the files_lock scalability work puts a
per-CPU list of open files into each superblock.  A CPU can be offlined
while there are open files in "its" lists, and nothing is done to shift
those files to a still-online CPU's list.  So there will still be
cross-CPU accesses to those lists as those files are closed; that means
we need to be sure to acquire locks associated with offline CPUs if we
want to avoid races.

lg_global_lock_online() is used (only) in the brlock implementation,
instead.  In this case, there's no leftover data if a CPU goes
offline, so no need to take locks associated with offline CPUs.

jon
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/