netdev - Re: [PATCH bpf v2] libbpf : make libbpf_num_possible

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Message-ID: <20190731153148.2e8bc1a7@cakuba.netronome.com>
Date:   Wed, 31 Jul 2019 15:31:48 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Takshak Chahande <ctakshak@...com>
Cc:     <netdev@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <rdna@...com>, <kernel-team@...com>, <hechaol@...com>
Subject: Re: [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function
 thread safe

On Wed, 31 Jul 2019 15:10:55 -0700, Takshak Chahande wrote:
> Having static variable `cpus` in libbpf_num_possible_cpus function
> without guarding it with mutex makes this function thread-unsafe.
> 
> If multiple threads accessing this function, in the current form; it
> leads to incrementing the static variable value `cpus` in the multiple
> of total available CPUs.
> 
> Used local stack variable to calculate the number of possible CPUs and
> then updated the static variable using WRITE_ONCE().
> 
> Changes since v1:
>  * added stack variable to calculate cpus
>  * serialized static variable update using WRITE_ONCE()
>  * fixed Fixes tag
> 
> Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
> Signed-off-by: Takshak Chahande <ctakshak@...com>

Perhaps we would have a little less code churn if the static variable
was renamed (e.g. to saved_cpus), but functionally looks good, so:

Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>

(FWIW I think Andrey's comment does not apply to the networking and BPF
trees so if you respin please keep the changelog in the commit message.)