linux-kernel - Re: [PATCH] x86/numa: Use cpumask_available instead of hardcoded NULL check

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Message-ID: <18263030220.8a990e82436929.3216192040531777412@siddh.me>
Date:   Wed, 03 Aug 2022 14:51:57 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     "Ingo Molnar" <mingo@...nel.org>
Cc:     "x86" <x86@...nel.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "linux-kernel-mentees" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: Re: [PATCH] x86/numa: Use cpumask_available instead of hardcoded
 NULL check

On Wed, 03 Aug 2022 14:38:19 +0530  Ingo Molnar <mingo@...nel.org> wrote:
> 
> * Siddh Raman Pant <code@...dh.me> wrote:
> > There is no reason why allmodconfig would trigger the warning, [...]
> 
> Well, unless I'm misreading your changelog, all the warning needs to 
> trigger is CONFIG_CPUMASK_OFFSTACK=y.
> 
> > as it has CONFIG_CPUMASK_OFFSTACK=y, but the warning is because of the 
> > other case.
> 
> What 'other case'? I've re-read the discussion and don't see it, but maybe 
> I'm a bit daft this morning ...
 
No, the warning is happening because CONFIG_CPUMASK_OFFSTACK is not set. This
is because in that case cpumask_var_t type is not a pointer, and thus the var
can never be NULL, which leads gcc to warn us when comparing with NULL.

The chain of events are like:

        #ifdef CONFIG_CPUMASK_OFFSTACK
                typedef struct cpumask *cpumask_var_t;
        #else
                typedef struct cpumask cpumask_var_t[1];
        endif

        cpumask_var_t node_to_cpumask_map[MAX_NUMNODES];
        ...
        if (node_to_cpumask_map[node] == NULL) {


The fix works because:

        #ifdef CONFIG_CPUMASK_OFFSTACK
                static inline bool cpumask_available(cpumask_var_t mask)
                {
                        return mask != NULL;
                }
        #else
                static inline bool cpumask_available(cpumask_var_t mask)
                {
                        return true;
                }
        endif


The conditional definitions, as pointed out earlier, can be seen from line 700
of include/linux/cpumask.h file.

> > Did you try the config file I had linked to earlier?
> 
> Yes, and it didn't trigger the warning.
> 
> Thanks,
> 
>     Ingo
> 

 Thanks,
 Siddh