linux-kernel - Re: [RFC] tcp: race in receive part

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Date:	Thu, 25 Jun 2009 12:51:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Jiri Olsa <jolsa@...hat.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, fbl@...hat.com, nhorman@...hat.com,
	davem@...hat.com
Subject: Re: [RFC] tcp: race in receive part

Oleg Nesterov a écrit :
> On 06/24, Oleg Nesterov wrote:
>> On 06/24, Jiri Olsa wrote:
>>> +/* The read_lock() on x86 is a full memory barrier. */
>>> +#define smp_mb__after_read_lock() barrier()
>> Just curious, why do we need barrier() ?
>>
>> I must admit, personally I dislike _read_lock part. Because I think we
>> need a "more generic" smp_mb__{before,after}_lock() or whatever which
>> work for spin_lock/read_lock/write_lock.
>>
>> In that case it can have more users. Btw, in fs/select.c too, see
>> __pollwake().
>>
>> And surprise,
>>
>>> --- a/fs/select.c
>>> +++ b/fs/select.c
>>> @@ -219,6 +219,10 @@ static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
>>>  	init_waitqueue_func_entry(&entry->wait, pollwake);
>>>  	entry->wait.private = pwq;
>>>  	add_wait_queue(wait_address, &entry->wait);
>>> +
>>> +	/* This memory barrier is paired with the smp_mb__after_read_lock
>>> +	 * in the sk_has_sleeper. */
>>> +	smp_mb();
>> This could be smp_mb__after_lock() too.
> 
> Cough. this needs mb__after_UNlock(), sorry.
> 

Yes, and this time you need separate smp_mb__after_spin_unlock(),
as rwlocks and spinlocks dont have same unlock implementation.

(spin_unlock dont have memory barrier on x86, while read_write_unlock do have a barrier)

As it wont give us a benefit on x86 but code obfuscation, I suspect we can leave this for now :)

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