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Date: Thu, 2 Jun 2022 23:04:18 +0200
From: Sander Vanheule <sander@...nheule.net>
To: Peter Zijlstra <peterz@...radead.org>,
Yury Norov <yury.norov@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Valentin Schneider <vschneid@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Marco Elver <elver@...gle.com>,
Barry Song <song.bao.hua@...ilicon.com>
Cc: linux-kernel@...r.kernel.org,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Sander Vanheule <sander@...nheule.net>
Subject: [PATCH v1 0/2] cpumask: Fix invalid uniprocessor assumptions
On uniprocessor builds, it is currently assumed that any cpumask will
contain the single CPU: cpu0. This assumption is used to provide
optimised implementations.
The current assumption also appears to be wrong, by ignoring the fact
that users can provide empty cpumask-s. This can result in bugs as
explained in [1].
This series seeks to fix this assumption, allowing for masks that
contain at most cpu0, i.e. are "1" or "0".
[1] https://lore.kernel.org/all/20220530082552.46113-1-sander@svanheule.net/
Best,
Sander
---
To test these changes, I used the following code:
static void cpumask_test(const struct cpumask *mask)
{
int cpu;
if (cpumask_empty(mask))
pr_info("testing empty mask\n");
else
pr_info("testing non-empty mask\n");
pr_info("first cpu: %d\n", cpumask_first(mask));
pr_info("first zero: %d\n", cpumask_first_zero(mask));
pr_info("first and: %d\n", cpumask_first_and(mask, cpu_possible_mask));
pr_info("last cpu: %d\n", cpumask_last(mask));
pr_info("next cpu at -1: %d\n", cpumask_next(-1, mask));
pr_info("next cpu at 0: %d\n", cpumask_next(0, mask));
pr_info("next zero at -1: %d\n", cpumask_next_zero(-1, mask));
pr_info("next zero at 0: %d\n", cpumask_next_zero(0, mask));
pr_info("next and at -1: %d\n", cpumask_next_and(-1, mask, cpu_possible_mask));
pr_info("next and at 0: %d\n", cpumask_next_and(0, mask, cpu_possible_mask));
pr_info("next wrap at -1,false: %d\n", cpumask_next_wrap(-1, mask, 0, false));
pr_info("next wrap at 0,false: %d\n", cpumask_next_wrap(-1, mask, 0, false));
pr_info("next wrap at -1,true: %d\n", cpumask_next_wrap(-1, mask, 0, true));
pr_info("next wrap at 0,true: %d\n", cpumask_next_wrap(-1, mask, 0, true));
for_each_cpu(cpu, mask)
pr_info("for each: %d\n", cpu);
for_each_cpu_not(cpu, mask)
pr_info("for each not: %d\n", cpu);
for_each_cpu_wrap(cpu, mask, 0)
pr_info("for each wrap: %d\n", cpu);
for_each_cpu_and(cpu, mask, cpu_possible_mask)
pr_info("for each and: %d\n", cpu);
}
static void run_tests()
{
cpumask_clear(&empty_mask);
cpumask_test(&empty_mask);
cpumask_test(cpu_possible_mask);
}
On an unpatched build, with NR_CPUS=1:
[...] testing empty mask
[...] first cpu: 0
[...] first zero: 0
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each not: 0
[...] for each wrap: 0
[...] for each and: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 0
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each not: 0
[...] for each wrap: 0
[...] for each and: 0
On a patched build, with NR_CPUS=1:
[...] testing empty mask
[...] first cpu: 1
[...] first zero: 0
[...] first and: 1
[...] last cpu: 1
[...] next cpu at -1: 1
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 1
[...] next and at 0: 1
[...] next wrap at -1,false: 1
[...] next wrap at 0,false: 1
[...] next wrap at -1,true: 1
[...] next wrap at 0,true: 1
[...] for each not: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 1
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 1
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each wrap: 0
For reference, the generic implementation with NR_CPUS=2, CONFIG_SMP=y
[...] testing empty mask
[...] first cpu: 2
[...] first zero: 0
[...] first and: 2
[...] last cpu: 2
[...] next cpu at -1: 2
[...] next cpu at 0: 2
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 2
[...] next and at 0: 2
[...] next wrap at -1,false: 2
[...] next wrap at 0,false: 2
[...] next wrap at -1,true: 2
[...] next wrap at 0,true: 2
[...] for each not: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 1
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 2
[...] next zero at -1: 1
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 2
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 2
[...] next wrap at 0,true: 2
[...] for each: 0
[...] for each wrap: 0
[...] for each and: 0
Sander Vanheule (2):
cpumask: Fix invalid uniprocessor mask assumption
cpumask: Add UP optimised for_each_*_cpu loops
include/linux/cpumask.h | 42 +++++++++++++++++++++--------------------
1 file changed, 22 insertions(+), 20 deletions(-)
--
2.36.1
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