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

Jonathan Corbet
2010-08-25T20:16:49Z

On Wed, 25 Aug 2010 13:00:59 -0700
Linus Torvalds <torvalds@linux-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