linux-kernel - Re: [PATCH 1/2] brw_mutex: big read-write mutex

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Date:	Wed, 17 Oct 2012 18:59:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex

On 10/16, Linus Torvalds wrote:
>
> On Mon, Oct 15, 2012 at 12:10 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > This patch adds the new sleeping lock, brw_mutex. Unlike rw_semaphore
> > it allows multiple writers too, just "read" and "write" are mutually
> > exclusive.
>
> So those semantics just don't sound sane. It's also not what any kind
> of normal "rw" lock ever does.

Yes, this is not usual.

And initially I made brw_sem which allows only 1 writer, but then
I changed this patch.

> So can you explain why these particular insane semantics are useful,
> and what for?

To allow multiple uprobe_register/unregister at the same time. Mostly
to not add the "regression", currently this is possible.

It is not that I think this is terribly important, but still. And
personally I think that "multiple writers" is not necessarily insane
in general. Suppose you have the complex object/subsystem, the readers
can use a single brw_mutex to access it "lockless", start_read() is
very cheap.

But start_write() is slow. Multiple writes can use the fine-grained
inside the start_write/end_write section and do not block each other.

Oleg.

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