linux-kernel - Re: bit fields && data tearing

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Date:	Tue, 23 Sep 2014 19:39:55 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	David Laight <David.Laight@...LAB.COM>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	linux-alpha@...r.kernel.org
Subject: Re: bit fields && data tearing

> > Yes - because if you think about it that tells you that nobody is hitting
> > it with the old code and it probably doesn't matter.
> 
> I don't understand this reply.

It's a matter of priorities. There are hundreds of potential security
holes turned up by scanners, 2,500+ filed bugs in kernel bugzilla alone.

Which matters more - fixing the bugs people hit or the ones that they
don't and which have a very high cost impact on other users ?

> Likewise, I'm pointing that byte-sized circular buffers will also be 
> corrupted under certain circumstances (when the head comes near the tail).

Yes. I believe the classic wording is "this problem has not been observed
in the field"
 
> 2. I'm not sure where you arrived at the conclusion that set_bit() et.al. is
> fast on x86. Besides the heavy-duty instructions (lgdt, mov cr0, etc), the
> bus-locked bit instructions are among the most expensive instructions in the
> kernel.

> I've attached a smoke test I use for lightweight validation of pty slave reads.
> If you run this smoke test under perf, almost every high-value hit is a
> bus-locked bit instruction. Check out canon_copy_from_read_buf() which performs
> the actual copy of data from the line discipline input buffer to the __user
> buffer; the clear_bit() is the performance hit. Check out cpu_idle_loop()...

And then go and instrument whether its the bit instructions or the cache
misses ?


If there is a significant performance improvement by spacing the fields
carefully, and you don't have other ordering problems as a result
(remembering that the compiler has a lot of re-ordering options provided
the re-ordering cannot be observed)

If that's the case then fine - submit a patch with numbers and the fields
spaced and quote the performance data.

Alan
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