linux-kernel - Re: [PATCH 1/5] ipc,shm: move BUG_ON check into shm

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Message-Id: <20150609152838.94774d7feafef3f7e6ccbd74@linux-foundation.org>
Date:	Tue, 9 Jun 2015 15:28:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/5] ipc,shm: move BUG_ON check into shm_lock

On Sat,  6 Jun 2015 06:37:56 -0700 Davidlohr Bueso <dave@...olabs.net> wrote:

> Upon every shm_lock call, we BUG_ON if an error was returned,
> indicating racing either in idr or in shm_destroy. Move this logic
> into the locking.
> 
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -155,8 +155,14 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
>  {
>  	struct kern_ipc_perm *ipcp = ipc_lock(&shm_ids(ns), id);
>  
> -	if (IS_ERR(ipcp))
> +	if (IS_ERR(ipcp)) {
> +		/*
> +		 * We raced in the idr lookup or with shm_destroy(),
> +		 * either way, the ID is busted.
> +		 */
> +		BUG();
>  		return (struct shmid_kernel *)ipcp;
> +	}

Was there any particular reason to still do it this way?  It's a bit
klunky.

--- a/ipc/shm.c~ipcshm-move-bug_on-check-into-shm_lock-fix
+++ a/ipc/shm.c
@@ -155,14 +155,11 @@ static inline struct shmid_kernel *shm_l
 {
 	struct kern_ipc_perm *ipcp = ipc_lock(&shm_ids(ns), id);
 
-	if (IS_ERR(ipcp)) {
-		/*
-		 * We raced in the idr lookup or with shm_destroy(),
-		 * either way, the ID is busted.
-		 */
-		BUG();
-		return (struct shmid_kernel *)ipcp;
-	}
+	/*
+	 * We raced in the idr lookup or with shm_destroy().  Either way, the
+	 * ID is busted.
+	 */
+	BUG_ON(IS_ERR(ipcp));
 
 	return container_of(ipcp, struct shmid_kernel, shm_perm);
 }

One benefit of the code you sent is that the unreachable `return' will
prevent a compile warning when CONFIG_BUG=n, but CONFIG_BUG=n is silly
and I never worry about it.
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