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

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Message-ID: <5618221F.9060205@hurleysoftware.com>
Date:	Fri, 9 Oct 2015 16:22:55 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Kosuke Tatsukawa <tatsu@...jp.nec.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

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.

Thanks again.

Regards,
Peter Hurley
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