From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Sat,  1 Mar 2014 23:05:19 -0500
To: 9fans@9fans.net
Message-ID: <0b927c978635da089774a72c0af3de19@mikro.quanstro.net>
In-Reply-To: <122a1b862278abf61a07a806168993ac@felloff.net>
References: <122a1b862278abf61a07a806168993ac@felloff.net>
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Subject: Re: [9fans] imagereclaim()
Topicbox-Message-UUID: c0c6d288-ead8-11e9-9d60-3106f5b1d025

On Sat Mar  1 22:56:25 EST 2014, cinap_lenrek@felloff.net wrote:
> checked nix/port/page.c. your duppage() is wrong.
>
> 	/* don't dup pages with no image */
> 	if(p->ref == 0 || p->image == nil || p->image->notext)
> 		return 0;
>
> 	/*
> 	 *  normal lock ordering is to call
> 	 *  lock(&pga) before lock(p).
> 	 *  To avoid deadlock, we have to drop
> 	 *  our locks and try again.
> 	 */
> 	if(!canlock(&pga)){
> 		unlock(p);
> 		if(up)
> 			sched();
> 		lock(p);
> 		goto retry;
> 	}
>
> you need to check p->ref != 1 instead of p->ref == 0. the page
> passed to duppage() is still cached. after you unlock(p), someone
> can come in and take a reference to the page from the image
> cache (lookpage()), making p->ref > 1 once you get the lock back.
>
> put an assert or print in there after the if(!canlock(&pga){} block
> to check p->ref.

why do i need an assert or print after the unlock?  it should go back
through the loop and notice p->ref != 1.

- erik