linux-kernel - Re: [PATCH] net: fix race in the receive/select

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Message-ID: <alpine.DEB.1.10.0906251858520.9517@makko.or.mcafeemobile.com>
Date:	Thu, 25 Jun 2009 19:04:04 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Oleg Nesterov <oleg@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH] net: fix race in the receive/select

On Fri, 26 Jun 2009, Eric Dumazet wrote:

> Davide Libenzi a écrit :
> > On Thu, 25 Jun 2009, Oleg Nesterov wrote:
> > 
> >> Can't really comment this patch, except this all looks reasonable to me.
> >> Add more CCs.
> > 
> > While this can work, IMO it'd be cleaner to have the smp_mb() moved from 
> > fs/select.c to the ->poll() function.
> > Having a barrier that matches another one in another susbsystem, because 
> > of the special locking logic of such subsystem, is not too shiny IMHO.
> > 
> 
> Yes but barrier is necessary only if add_wait_queue() was actually called, and __pollwait()
> does this call.
> 
> Adding a plain smp_mb() in tcp_poll() for example would slowdown select()/poll() with NULL
> timeout.

Do you think of it as good design adding an MB on a subsystem, because of 
the special locking logic of another one?
The (eventual) slowdown, IMO can be argued sideways, by saying that 
non-socket users will pay the price for their polls.



- Davide