linux-kernel - Re: md: dm-writeback: add __noreturn to BUG-ging function

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Date:   Wed, 18 Nov 2020 16:24:38 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Mike Snitzer <snitzer@...hat.com>
cc:     Randy Dunlap <rdunlap@...radead.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-kernel@...r.kernel.org, Alasdair Kergon <agk@...hat.com>,
        dm-devel@...hat.com, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: md: dm-writeback: add __noreturn to BUG-ging function



On Wed, 18 Nov 2020, Mike Snitzer wrote:

> On Wed, Nov 18 2020 at 10:49am -0500,
> Mike Snitzer <snitzer@...hat.com> wrote:
> 
> > I don't think my suggestion will help.. given it'd still leave
> > persistent_memory_claim() without a return statement.
> > 
> > Think it worthwhile to just add a dummy 'return 0;' after the BUG().
> 
> Decided to go with this, now staged for 5.11:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.11&id=a1e4865b4dda7071f3707f7e551289ead66e38b1

Hi

I would just use "return -EOPNOTSUPP;" and drop the "#ifdef 
DM_WRITECACHE_HAS_PMEM" that you added.

That BUG/return -EOPNOTSUPP code can't happen at all - if 
DM_WRITECACHE_HAS_PMEM is not defined, WC_MODE_PMEM(wc) always returns 
false - so persistent_memory_claim and BUG() can't ever be called. And if 
it can't be called, you don't need to add a code that prints an error in 
that case.

If we don't have DM_WRITECACHE_HAS_PMEM, the compiler optimizer will 
remove all the code guarded with if (WC_MODE_PMEM(wc)) as unreachable.

Mikulas


From: Mikulas Patocka <mpatocka@...hat.com>
Subject: [PATCH] dm writecache: remove BUG() and fail gracefully insteadfor-nextdm-5.11

Building on arch/s390/ results in this build error:

cc1: some warnings being treated as errors
../drivers/md/dm-writecache.c: In function 'persistent_memory_claim':
../drivers/md/dm-writecache.c:323:1: error: no return statement in function returning non-void [-Werror=return-type]

Fix this by replacing the BUG() with a -EOPNOTSUPP return.

Fixes: 48debafe4f2f ("dm: add writecache target")
Cc: stable@...r.kernel.org	# v4.18+
Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

Index: linux-2.6/drivers/md/dm-writecache.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-writecache.c
+++ linux-2.6/drivers/md/dm-writecache.c
@@ -319,7 +319,7 @@ err1:
 #else
 static int persistent_memory_claim(struct dm_writecache *wc)
 {
-	BUG();
+	return -EOPNOTSUPP;
 }
 #endif