linux-kernel - Re: [PATCH v2] tty: fix stall caused by missing memory barrier in drivers/tty/n

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Message-ID: <17EC94B0A072C34B8DCF0D30AD16044A02875FE3@BPXM09GP.gisp.nec.co.jp>
Date:	Sat, 10 Oct 2015 05:04:39 +0000
From:	Kosuke Tatsukawa <tatsu@...jp.nec.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tty: fix stall caused by missing memory barrier in
 drivers/tty/n_tty.c 

Peter Hurley wrote:
> On 10/09/2015 01:28 PM, Peter Hurley wrote:
>> Tatsukawa-san,
>> 
>> I would still like to root-cause the reported stall; is the reported
>> stall resolved if smp_mb() is added before the waitqueue_active()
>> in __receive_buf()?
>
> Nevermind, I see it now.
>
> The store to commit_head is deferred until after the load of read_wait->next;
> a full memory barrier would properly order the store before the load but,
> since that is roughly equivalent to taking the spin lock for the wake up anyway,
> it makes sense to just always do the wakeup.

The stall problem is resolved if smp_mb() is added both before the
waitqueue_active() in __receive_buf(), and before the line containing
input_available_p() in n_tty_read().
---
Kosuke TATSUKAWA  | 3rd IT Platform Department
                  | IT Platform Division, NEC Corporation
                  | tatsu@...jp.nec.com
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