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From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>,
	leonardo.macchia@gmail.com
Subject: Re: [PATCH 1/3] inet: Fix getnameinfo (NI_NOFQDN) race condition (BZ#28566)
Date: Thu, 11 Nov 2021 11:09:51 -0300	[thread overview]
Message-ID: <e7b14ba5-2b73-1431-dbc8-c3dc4f3d1253@linaro.org> (raw)
In-Reply-To: <87czn6yf0d.fsf@oldenburg.str.redhat.com>



On 11/11/2021 10:54, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> On 11/11/2021 05:16, Florian Weimer wrote:
>>> * Adhemerval Zanella via Libc-alpha:
>>>
>>>> diff --git a/inet/getnameinfo.c b/inet/getnameinfo.c
>>>> index 8380d85783..58ebbb1154 100644
>>>> --- a/inet/getnameinfo.c
>>>> +++ b/inet/getnameinfo.c
>>>> @@ -86,55 +86,75 @@ libc_freeres_ptr (static char *domain);
>>>>  static char *
>>>>  nrl_domainname (void)
>>>>  {
>>>> +  __libc_lock_define_initialized (static, lock);
>>>> +  __libc_lock_lock (lock);
>>>>  
>>>> +  static bool not_first = false;
>>>>    if (! not_first)
>>>
>>>> +    done:
>>>> +      scratch_buffer_free (&tmpbuf);
>>>> +      not_first = true;
>>>
>>> This is missing the acquire/release pairing for the double-checked
>>> locking idiom.  You can probably use the domain variable directly.
>>
>> But it is done now within the lock, different than current implementation
>> which does outside.  I moved to be within the lock exactly to avoid the
>> double-checked locking idiom.
> 
> Ah, sorry, I had missed that.
> 
>> I think now that we might be moving to a more optimized lll_lock internally 
>> using a acquire-load+CAS instead of just CAS we can get it without need 
>> to code it explicitly.
> 
> The double-checked locking idiom avoids the CAS after initialization.
> With the lll_lock change, an atomic read-modify-write operation still
> happens on the lock in all cases (prior to the eventual return to the
> caller).

I meant H.J internal lock optimization [1], where the lll_lock int this
case will be mostly a relaxed load instead of a CAS (since once 'domain'
is properly initialized, there is no need to take the lock).

[1] https://sourceware.org/pipermail/libc-alpha/2021-November/132899.html

  reply	other threads:[~2021-11-11 14:09 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-10 18:58 [PATCH 0/3] Fixes for getnameinfo() with NI_NOFQDN Adhemerval Zanella
2021-11-10 18:58 ` [PATCH 1/3] inet: Fix getnameinfo (NI_NOFQDN) race condition (BZ#28566) Adhemerval Zanella
2021-11-11  8:16   ` Florian Weimer
2021-11-11 13:34     ` Adhemerval Zanella
2021-11-11 13:54       ` Florian Weimer
2021-11-11 14:09         ` Adhemerval Zanella [this message]
2021-11-11 14:12           ` Florian Weimer
2021-11-11 14:17             ` Adhemerval Zanella
2021-11-11 17:41               ` Florian Weimer
2021-11-10 18:58 ` [PATCH 2/3] inet: Remove strdupa from nrl_domainname() Adhemerval Zanella
2021-11-11  8:18   ` Florian Weimer
2021-11-11 13:36     ` Adhemerval Zanella
2021-11-11 14:00       ` Florian Weimer
2021-11-10 18:58 ` [PATCH 3/3] inet: Return EAI_MEMORY when nrl_domainname() fails to allocate memory Adhemerval Zanella

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