linux-kernel - Re: rpc_xprt_debugfs_register()

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Date:   Thu, 21 Jan 2021 11:01:09 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Trond Myklebust <trondmy@...merspace.com>,
        "Anna.Schumaker@...app.com" <Anna.Schumaker@...app.com>
Cc:     "bfields@...ldses.org" <bfields@...ldses.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "chuck.lever@...cle.com" <chuck.lever@...cle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: rpc_xprt_debugfs_register() - atomic_inc_return() usage

On 1/21/21 10:56 AM, Trond Myklebust wrote:
> On Wed, 2021-01-20 at 16:52 -0700, Shuah Khan wrote:
>> Hi Anna and Trond,
>>
>> I came across the following while reviewing atomic_inc_return()
>> usages
>> that cast return value to unsigned
>>
>> rpc_xprt_debugfs_register()'s atomic_inc_return() usage looks a bit
>> odd.
>>
>> - cur_id isn't initialized
>> - id = (unsigned int)atomic_inc_return(&cur_id);
>>
>> Please note that id is int. Is it expected that cur_id could
>> overflow?
>> Is there a maximum limit for this value?
>>
> 
> Yes, we do expect cur_id to eventually overflow (once you have created
> 2 billion RPC client instances), however the atomic increment
> operations are expected to handle this correctly according to the
> maintainers (I already asked them in a different context). Furthermore,
> the code itself doesn't care about strict sequentiality. All it wants
> from the counter is uniqueness, with that uniqueness condition actually
> being enforced by the subsequent debugfs_create_file() call.
> 
> IOW: I don't think this is a real problem.
> 

Great. Thank you for a detailed explanation.

-- Shuah