From: Paul Mackerras <paulus@ozlabs.org>
To: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org,
"linuxppc-dev@ozlabs.org" <linuxppc-dev@ozlabs.org>,
"linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>
Subject: Re: [PATCH] bitmap: Fix optimization of bitmap_set/clear for big-endian machines
Date: Wed, 25 Oct 2017 19:46:02 +1100 [thread overview]
Message-ID: <20171025084602.GC25140@fergus.ozlabs.ibm.com> (raw)
In-Reply-To: <MWHPR21MB08455F1374F717DBA20B7798CB440@MWHPR21MB0845.namprd21.prod.outlook.com>
On Wed, Oct 25, 2017 at 07:39:48AM +0000, Matthew Wilcox wrote:
> Hang on, don't tell me you found this by inspection. Are you not running the bitmap testcase, enabled by CONFIG_TEST_BITMAP? Either that should be producing an error, or there's a missing test case, or your inspection is wrong ...
I did find it by inspection. I was looking for a version of the
bitmap_* API that does little-endian style bitmaps on all systems, and
the inline bitmap_set() does that in the case where it calls memset,
but not in the case where it calls __bitmap_set.
I'll fire up a big-endian system tomorrow when I get to work to run
the test case. (PPC64 is almost entirely little-endian these days as
far as the IBM POWER systems are concerned.)
In any case, it's pretty clearly wrong as it is. On a big-endian
64-bit system, bitmap_set(p, 56, 16) should set bytes 0 and 15 to
0xff, and there's no way a single memset can do that.
Paul.
(and yes, I stuffed up the address for lkml)
next prev parent reply other threads:[~2017-10-25 8:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-25 6:57 [PATCH] bitmap: Fix optimization of bitmap_set/clear for big-endian machines Paul Mackerras
2017-10-25 7:39 ` Matthew Wilcox
2017-10-25 8:46 ` Paul Mackerras [this message]
2017-10-25 9:11 ` Matthew Wilcox
2017-10-25 10:30 ` Michael Ellerman
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