linux-kernel - Re: [EXT] Re: [PATCH 1/2] net/iucv: Improve unlocking in iucv

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Date: Tue, 2 Jan 2024 10:53:15 +0100
From: Alexandra Winter <wintera@...ux.ibm.com>
To: Suman Ghosh <sumang@...vell.com>, Markus Elfring <Markus.Elfring@....de>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Wenjia Zhang <wenjia@...ux.ibm.com>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: [EXT] Re: [PATCH 1/2] net/iucv: Improve unlocking in
 iucv_enable()



On 02.01.24 09:27, Suman Ghosh wrote:
>>> [Suman] This looks confusing. What is the issue with retaining the
>> original change?
>>
>> I propose to reduce the number of cpus_read_unlock() calls (in the
>> source code).
>>
>> Regards,
>> Markus
> [Suman] Then I think we should do something like this. Changing the code flow back-and-forth using "goto" does not seem correct.

I share Suman's concern that jumping backwards goto is confusing.
But I think the Coccinelle finding of freeing a null-pointer should be addressed (see patch 2/2)
Thank you Markus for reporting it.

The allocation does require holding the cpus_read_lock. 
For some reason Markus wants to reduce the number of cpus_read_unlock() calls (why?),
so what about something like this for both issues:

diff --git a/net/iucv/iucv.c b/net/iucv/iucv.c
index 0ed6e34d6edd..1030403b826b 100644
--- a/net/iucv/iucv.c
+++ b/net/iucv/iucv.c
@@ -542,24 +542,22 @@ static int iucv_enable(void)
        size_t alloc_size;
        int cpu, rc;

-       cpus_read_lock();
-       rc = -ENOMEM;
        alloc_size = iucv_max_pathid * sizeof(struct iucv_path);
        iucv_path_table = kzalloc(alloc_size, GFP_KERNEL);
        if (!iucv_path_table)
-               goto out;
+               return -ENOMEM;
        /* Declare per cpu buffers. */
-       rc = -EIO;
+       cpus_read_lock();
        for_each_online_cpu(cpu)
                smp_call_function_single(cpu, iucv_declare_cpu, NULL, 1);
-       if (cpumask_empty(&iucv_buffer_cpumask))
+       if (cpumask_empty(&iucv_buffer_cpumask)) {
                /* No cpu could declare an iucv buffer. */
-               goto out;
-       cpus_read_unlock();
-       return 0;
-out:
-       kfree(iucv_path_table);
-       iucv_path_table = NULL;
+               kfree(iucv_path_table);
+               iucv_path_table = NULL;
+               rc = -EIO;
+       } else {
+               rc = 0;
+       }
        cpus_read_unlock();
        return rc;
 }