Re: [RFC v0 1/1] fs/locks: Use plain percpu spinlocks instead of lglock to protect file_lock
From: Daniel Wagner
Date: Fri Feb 20 2015 - 09:39:27 EST
On 02/19/2015 09:49 PM, Jeff Layton wrote:
> On Thu, 19 Feb 2015 15:26:14 +0100
> Daniel Wagner <daniel.wagner@xxxxxxxxxxxx> wrote:
>> Whenever we insert a new lock we are going to grab besides the i_lock
>> also the corresponding percpu file_lock_lock. The global
>> blocked_lock_lock is only used when blocked_hash is involved.
> Ok, good. I like that part -- I hate the blocked_lock_lock, but freezing
> the file locking state is the only way I've found to ensure the
> correctness of deadlock detection. Bummer.
Okay, I'll look into that.
>> file_lock_list exists to be being able to produce the content of
>> /proc/locks. For listing the all locks it seems a bit excessive to
>> grab all locks at once. We should be okay just grabbing the
>> corresponding lock when iterating over the percpu file_lock_list.
> True, but that's not a terribly common event, which is why I figured
> the lg_lock was an acceptable tradeoff there. That said, if you can get
> rid of it in favor of something more efficient then that sounds good to
> me. If it helps the -rt kernels, then so much the better...
Great! I was hoping to hear that :)
>> file_lock_lock protects now file_lock_list and fl_link, fl_block and
>> fl_next allone. That means we need to define which file_lock_lock is
>> used for all waiters. Luckely, fl_link_cpu can be reused for fl_block
>> and fl_next.
> Ok, so when a lock is blocked, we'll insert the waiter onto the
> fl_block list for the blocker, and use the blocker's per-cpu spinlock
> to protect that list. Good idea.
Let's hope it doesn't explode. So far I am still confident it works.
>> Signed-off-by: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx>
>> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Cc: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
>> Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
>> Cc: John Kacur <jkacur@xxxxxxxxxx>
>> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
>> Cc: linux-rt-users@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Thanks for the patch. Some general comments first:
> - fs/locks.c has seen a fair bit of overhaul during the current merge
> window, and this patch won't apply directly. I'd suggest cleaning it up
> after -rc1 is cut.
I just rebased it and splited it a bit up. Couldn't wait...
> - the basic idea seems sound, but this is very "fiddly" code. It would
> be nice to see if you can break this up into multiple patches. For
> instance, maybe convert the lglock to the percpu spinlocks first in a
> separate patch, and just keep it protecting the file_lock_list. Once
> that's done, start changing other pieces to be protected by the percpu
> locks. Small, incremental changes like that are much easier to deal
> with if something breaks, since we can at least run a bisect to narrow
> down the offending change. They're also easier to review.
I complete agree. Sorry to send such a bad initial version. I should
have known it better.
> - the open-coded seqfile stuff is pretty nasty. When I did the original
> change to use lglocks, I ended up adding seq_hlist_*_percpu macros to
> support it. Maybe consider doing something like that here too, though
> this is a bit more complex obviously since you want to be able to just
> hold one spinlock at a time.
> - it would also be good to start thinking about adding lockdep
> assertions to this code. I simply didn't realize how wonderful they
> were when I did the global spinlock to i_lock conversion a year or two
> ago. That can really help catch bugs, and as the (spin)locking gets
> more complex in this code, that'd be good to help ensure correctness.
I'll give it a try.
Many thanks for the quick review. Really appreciated!
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/