linux-kernel - Re: ipc,sem: sysv semaphore scalability

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Message-ID: <CA+55aFwF7QxH3i+nSH4uw7dZ8+cU=PdndEd+Jidt2zp2RNSkcg@mail.gmail.com>
Date:	Fri, 29 Mar 2013 12:54:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	hhuang@...hat.com, "Low, Jason" <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>
Subject: Re: ipc,sem: sysv semaphore scalability

On Fri, Mar 29, 2013 at 12:33 PM, Peter Hurley <peter@...leysoftware.com> wrote:
> On Fri, 2013-03-29 at 11:43 -0700, Linus Torvalds wrote:
>> I think I foud at least one bug in the MSG_COPY stuff: it leaks the
>> "copy" allocation if
>>
>>     mode == SEARCH_LESSEQUAL
>>
>> but maybe I'm misreading it.
>
> Yes, you're misreading it. copy_msg() returns the 'copy' address when
> copying is successful. So this patch double-frees 'copy'.

No it doesn't.

The case where "copy_msg()" *is* called and is successful, msg gets
set to "copy", my patch sets "copy" to NULL, so no, it doesn't
double-free.

That said, it looks like if MSG_COPY is set, we cannot trigger the
"mode == SEARCH_LESSEQUAL" case, because it forces msgtyp to 0, which
in turn forces mode = SEARCH_ANY.

So the actual leak case seems to not be possible, although that's
*really* hard to see from looking at the code. The code is just an
unreadable mess.

              Linus
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