linux-kernel - Re: [PATCH v4 25/32] tty: Add read-recursive, writer-prioritized rw semaphore

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Date:	Fri, 08 Mar 2013 12:12:42 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, Sasha Levin <levinsasha928@...il.com>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v4 25/32] tty: Add read-recursive, writer-prioritized rw
 semaphore

On Wed, 2013-02-20 at 15:03 -0500, Peter Hurley wrote:
> The semantics of a rw semaphore are almost ideally suited
> for tty line discipline lifetime management;  multiple active
> threads obtain "references" (read locks) while performing i/o
> to prevent the loss or change of the current line discipline
> (write lock).
> 
> Unfortunately, the existing rw_semaphore is ill-suited in other
> ways;
> 1) obtaining a "reference" can be recursive, ie., a reference holder
>    may attempt to obtain another "reference". Recursive read locks
>    are not supported by rwsem.

Hi Greg,

I just finished auditing all the recursive usage of the tty/ldisc layer
and I can make this lock non-recursive.

Please hold off applying this and I'll send you a v5 series in a couple
of days, ok?

Regards,
Peter Hurley


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