From: green@linuxhacker.ru
To: Rusty Russell <rusty@rustcorp.com.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        "David S. Miller" <davem@davemloft.net>
Cc: linux-kernel@vger.kernel.org, Oleg Drokin <green@linuxhacker.ru>
Subject: [PATCH 0/2] incorrect cpumask behavior with CPUMASK_OFFSTACK
Date: Thu, 26 Feb 2015 01:19:09 -0500
Message-Id: <1424931551-11757-1-git-send-email-green@linuxhacker.ru>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2571
Lines: 62

From: Oleg Drokin <green@linuxhacker.ru>

I just got a report today from Tyson Whitehead <twhitehead@gmail.com>
that Lustre crashes when CPUMASK_OFFSTACK is enabled.

A little investigation revealed that this code:
        cpumask_t                       mask;
...
        cpumask_copy(&mask, topology_thread_cpumask(0));
        weight = cpus_weight(mask);

that was supposed to calculate number of cpu siblings/partitions returns
a crazy high number over 3000 which is impossible as I only have
8 cpu cores on my system.

So after a bit of digging I found out that:
cpumask_copy only copies up to nr_cpumask_bits (actual number of cpus I
have in the system),
where as cpumask_weight actully tries to count bits up to NR_CPUS.

Not only calculating up to NR_CPUS is wasteful in this case, and
since we know how many cpus we have in the system - it only makes sense
to calculate only this much anyway, it's wrong because the copy only copied
8 bits to our variable and the rest of it is some random stack garbage.

So I propose two patches here, the first one I am more certain about -
operations that operate on current cpuset like cpus_weight, but also
cpus_empty, cpus_$LOGICALop cpus_$BINARYop are converted from NR_CPUS to
nr_cpumask_bits (this is ok when CONFIG_CPUMASK_OFFSTACK is not set as it's
then defined to NR_CPUS anyway).
I am leaving __cpus_setall __cpus_clear out of it as these two look like
they deal with entire set and it would be useful for them to operate on
all NR_CPUS bits for the case if more cpus are added later and such.

The second patch that I am not sure if we wnat, but it seems to be useful
until struct cpumask is fully dynamic is to convert what looks like
whole-set operations e.g. copies, namely:
cpumask_setall, cpumask_clear, cpumask_copy to always operate on NR_CPUS
bits to ensure there's no stale garbage left in the mask should the
cpu count increases later.

I checked the code and allocating cpumasks on stack is not all that 
uncommon in the code, so this should be a worthwhile fix.

Please consider.

Oleg Drokin (2):
  cpumask: Properly calculate cpumask values
  cpumask: make whole cpumask operations like copy to work with NR_CPUS
    bits

 include/linux/cpumask.h | 37 +++++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 16 deletions(-)

-- 
2.1.0

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