Date: Wed, 21 Dec 2005 16:57:42 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: linux-kernel@vger.kernel.org, Linus Torvalds <torvalds@osdl.org>,
       Andrew Morton <akpm@osdl.org>, Arjan van de Ven <arjanv@infradead.org>,
       Steven Rostedt <rostedt@goodmis.org>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Christoph Hellwig <hch@infradead.org>, Andi Kleen <ak@suse.de>,
       David Howells <dhowells@redhat.com>,
       Alexander Viro <viro@ftp.linux.org.uk>, Paul Jackson <pj@sgi.com>
Subject: Re: [patch 05/15] Generic Mutex Subsystem, mutex-core.patch
Message-ID: <20051221155742.GA7375@elte.hu>
References: <20051219013718.GA28038@elte.hu> <43A98101.364DB5CF@tv-sign.ru>
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* Oleg Nesterov <oleg@tv-sign.ru> wrote:

> > +	spin_lock(&lock->wait_lock);
> > +	__add_waiter(lock, waiter, ti, task __IP__);
> > +	set_task_state(task, task_state);
> 
> I can't understand why __mutex_lock_common() does xchg() after adding 
> the waiter to the ->wait_list. We are holding ->wait_lock, we can't 
> race with __mutex_unlock_nonatomic() - it calls wake_up() and sets 
> ->count under this spinlock.

we must make sure that the drop has not been dropped meanwhile, on the 
way in, between the fastpath-unlock atomic op, and the xchg() here.

	Ingo
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