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

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Date:	Wed, 24 Jun 2009 18:41:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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

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.

Oleg.

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