linux-kernel - [PATCH] cpumask and md/raid5: Fix implicit type conversion

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Date:   Tue, 26 Oct 2021 09:26:23 +0000
From:   Jiasheng Jiang <jiasheng@...as.ac.cn>
To:     song@...nel.org, valentin.schneider@....com, peterz@...radead.org,
        mingo@...nel.org, dave.hansen@...ux.intel.com, bristot@...hat.com,
        namit@...are.com
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jiasheng Jiang <jiasheng@...as.ac.cn>
Subject: [PATCH] cpumask and md/raid5: Fix implicit type conversion

The description of the macro in `include/linux/cpumask.h` says the
variable 'cpu' can be unsigned int.
However in the for_each_cpu(), for_each_cpu_wrap() and
for_each_cpu_and(), its value is assigned to -1.
That doesn't make sense. Moreover in the cpumask_next(),
cpumask_next_zero(), cpumask_next_wrap() and cpumask_next_and(),
'cpu' will be implicitly type conversed to int if the type is
unsigned int.
It is universally accepted that the implicit type conversion is
terrible.
Also, having the good programming custom will set an example for
others.
Thus, it might be better to fix the macro description of 'cpu' that
remove the '(optionally unsigned)' and change the definition of 'cpu'
in `drivers/md/raid5.c` from unsigned long to long.

Fixes: c743f0a ("sched/fair, cpumask: Export for_each_cpu_wrap()")
Fixes: 8bd93a2 ("rcu: Accelerate grace period if last non-dynticked CPU")
Fixes: 984f2f3 ("cpumask: introduce new API, without changing anything, v3")
Fixes: 738a273 ("md/raid5: fix allocation of 'scribble' array.")
Signed-off-by: Jiasheng Jiang <jiasheng@...as.ac.cn>
---
 drivers/md/raid5.c      | 2 +-
 include/linux/cpumask.h | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 7d4ff8a..32ef82b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2425,7 +2425,7 @@ static int scribble_alloc(struct raid5_percpu *percpu,
 
 static int resize_chunks(struct r5conf *conf, int new_disks, int new_sectors)
 {
-	unsigned long cpu;
+	long cpu;
 	int err = 0;
 
 	/*
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index bfc4690..ceaed99 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -232,7 +232,7 @@ int cpumask_any_distribute(const struct cpumask *srcp);
 
 /**
  * for_each_cpu - iterate over every cpu in a mask
- * @cpu: the (optionally unsigned) integer iterator
+ * @cpu: the integer iterator
  * @mask: the cpumask pointer
  *
  * After the loop, cpu is >= nr_cpu_ids.
@@ -244,7 +244,7 @@ int cpumask_any_distribute(const struct cpumask *srcp);
 
 /**
  * for_each_cpu_not - iterate over every cpu in a complemented mask
- * @cpu: the (optionally unsigned) integer iterator
+ * @cpu: the integer iterator
  * @mask: the cpumask pointer
  *
  * After the loop, cpu is >= nr_cpu_ids.
@@ -258,7 +258,7 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool
 
 /**
  * for_each_cpu_wrap - iterate over every cpu in a mask, starting at a specified location
- * @cpu: the (optionally unsigned) integer iterator
+ * @cpu: the integer iterator
  * @mask: the cpumask poiter
  * @start: the start location
  *
@@ -273,7 +273,7 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool
 
 /**
  * for_each_cpu_and - iterate over every cpu in both masks
- * @cpu: the (optionally unsigned) integer iterator
+ * @cpu: the integer iterator
  * @mask1: the first cpumask pointer
  * @mask2: the second cpumask pointer
  *
-- 
2.7.4