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Date: Thu, 31 Jan 2002 10:48:32 -0800
From: Alfred Perlstein <alfred@freebsd.org>
To: Bruce Evans <bde@zeta.org.au>
Cc: current@freebsd.org
Subject: Re: lock reversal in fdalloc()
Message-ID: <20020131104831.E18604@elvis.mu.org>
References: <20020201043751.O5590-100000@gamplex.bde.org>
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* Bruce Evans <bde@zeta.org.au> [020131 09:42] wrote:
> Jan 31 18:27:29 gamplex kernel: lock order reversal
> Jan 31 18:27:29 gamplex kernel: 1st 0xc26ea034 filedesc structure @ ./@/kern/kern_descrip.c:925
> Jan 31 18:27:29 gamplex kernel: 2nd 0xc031eca0 Giant @ ./@/kern/kern_descrip.c:959
> 
> %%%
> Index: kern_descrip.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/kern/kern_descrip.c,v
> retrieving revision 1.122
> diff -u -2 -r1.122 kern_descrip.c
> --- kern_descrip.c	29 Jan 2002 22:54:19 -0000	1.122
> +++ kern_descrip.c	31 Jan 2002 07:32:43 -0000
> @@ -957,7 +967,9 @@
>  		fdexpand++;
>  		if (oldofile != NULL) {
> +			FILEDESC_UNLOCK(fdp);
>  			mtx_lock(&Giant);
>  			FREE(oldofile, M_FILEDESC);
>  			mtx_unlock(&Giant);
> +			FILEDESC_LOCK(fdp);
>  		}
>  	}
> %%%
> 
> I'm not sure that releasing the lock here is safe, but other parts of
> fdalloc() do this.

I don't think this is safe at a glance, I think it's only safe right
before return'ing from the function.  I'll look at it later tonight.

-Alfred

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