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From:   Nadav Amit <namit@vmware.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Rik van Riel <riel@surriel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH v5 1/8] smp: Run functions concurrently in
 smp_call_function_many_cond()
Thread-Topic: [PATCH v5 1/8] smp: Run functions concurrently in
 smp_call_function_many_cond()
Thread-Index: AQHXBFvlshCJ6CO6YEWdmGpC4cHblapbIDAA
Date:   Tue, 16 Feb 2021 18:49:28 +0000
Message-ID: <AE387DFD-770B-47EB-AF85-4AB8950D8ABF@vmware.com>
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 <20210209221653.614098-2-namit@vmware.com>
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> On Feb 16, 2021, at 4:04 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> On Tue, Feb 09, 2021 at 02:16:46PM -0800, Nadav Amit wrote:
>> @@ -894,17 +911,12 @@ EXPORT_SYMBOL(on_each_cpu_mask);
>> void on_each_cpu_cond_mask(smp_cond_func_t cond_func, smp_call_func_t fu=
nc,
>> 			   void *info, bool wait, const struct cpumask *mask)
>> {
>> -	int cpu =3D get_cpu();
>> +	unsigned int scf_flags =3D SCF_RUN_LOCAL;
>>=20
>> -	smp_call_function_many_cond(mask, func, info, wait, cond_func);
>> -	if (cpumask_test_cpu(cpu, mask) && cond_func(cpu, info)) {
>> -		unsigned long flags;
>> +	if (wait)
>> +		scf_flags |=3D SCF_WAIT;
>>=20
>> -		local_irq_save(flags);
>> -		func(info);
>> -		local_irq_restore(flags);
>> -	}
>> -	put_cpu();
>> +	smp_call_function_many_cond(mask, func, info, scf_flags, cond_func);
>> }
>> EXPORT_SYMBOL(on_each_cpu_cond_mask);
>=20
> You lost the preempt_disable() there, I've added it back:
>=20
> ---
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -920,7 +920,9 @@ void on_each_cpu_cond_mask(smp_cond_func
> 	if (wait)
> 		scf_flags |=3D SCF_WAIT;
>=20
> +	preempt_disable();
> 	smp_call_function_many_cond(mask, func, info, scf_flags, cond_func);
> +	preempt_enable();
> }
> EXPORT_SYMBOL(on_each_cpu_cond_mask);

Indeed. I will add lockdep_assert_preemption_disabled() to
smp_call_function_many_cond() to prevent this mistake from reoccurring.