linux-kernel - [RFC] issue with cpumask for UniProcessor

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Date:   Mon,  8 Aug 2022 09:23:41 -0700
From:   Saurabh Sengar <ssengar@...ux.microsoft.com>
To:     ssengar@...rosoft.com, mikelley@...rosoft.com,
        sander@...nheule.net, linux-kernel@...r.kernel.org
Subject: [RFC] issue with cpumask for UniProcessor

Hi,

I am working on a UniProcessor system with latest linux-next kernel (20220803).
I observed two files "shared_cpu_map” and “shared_cpu_list” are missing
for L3 cache (/sys/devices/system/cpu/cpu0/cache/index3). This causes lscpu
version 2.34 to segfault. On further digging I figured below is the commit
which introduced this problem.

https://lore.kernel.org/lkml/e78c55ecb98172356248a7a89da501479ead6ae0.1659077534.git.sander@svanheule.net/

I am not 100% certain what the proper fix for it is, but below changes fix
this issue. I understand above patch is already confirmed for linux kernel
6.0, please suggest if we need fixing this in 6.0.

Regards,
Saurabh

diff --git a/lib/cpumask.c b/lib/cpumask.c
index b9728513a4d4..81fc2e35b5b1 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -16,10 +16,14 @@
  */
 unsigned int cpumask_next(int n, const struct cpumask *srcp)
 {
+#if NR_CPUS == 1
+       return n+1;
+#else
        /* -1 is a legal arg here. */
        if (n != -1)
                cpumask_check(n);
        return find_next_bit(cpumask_bits(srcp), nr_cpumask_bits, n + 1);
+#endif
 }
 EXPORT_SYMBOL(cpumask_next);