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]
Date:	Thu, 3 Jan 2013 14:48:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, zab@...hat.com, bcrl@...ck.org,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk,
	tytso@....edu
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting

On Wed, 26 Dec 2012 18:00:02 -0800
Kent Overstreet <koverstreet@...gle.com> wrote:

> This implements a refcount with similar semantics to
> atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t
> but dynamically switches to per cpu refcounting when the rate of
> gets/puts becomes too high.
> 
> It also implements two stage shutdown, as we need it to tear down the
> percpu counts. Before dropping the initial refcount, you must call
> percpu_ref_kill(); this puts the refcount in "shutting down mode" and
> switches back to a single atomic refcount with the appropriate barriers
> (synchronize_rcu()).
> 
> It's also legal to call percpu_ref_kill() multiple times - it only
> returns true once, so callers don't have to reimplement shutdown
> synchronization.
> 
> For the sake of simplicity/efficiency, the heuristic is pretty simple -
> it just switches to percpu refcounting if there are more than x gets
> in one second (completely arbitrarily, 4096).
> 
> It'd be more correct to count the number of cache misses or something
> else more profile driven, but doing so would require accessing the
> shared ref twice per get - by just counting the number of gets(), we can
> stick that counter in the high bits of the refcount and increment both
> with a single atomic64_add(). But I expect this'll be good enough in
> practice.

I still don't "get" why this code exists.  It is spectacularly,
stunningly undocumented and if someone were to ask me "under what
circumstances should I use percpu-refcount", I would not be able to
help them.

--
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/

Powered by blists - more mailing lists