From: Linus Torvalds <torvalds@linux-foundation.org> To: Konstantin Khlebnikov <khlebnikov@openvz.org> Cc: Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Minchan Kim <minchan@kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Ben Herrenschmidt <benh@kernel.crashing.org>, "linux@arm.linux.org.uk" <linux@arm.linux.org.uk> Subject: Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit Date: Thu, 22 Mar 2012 15:39:00 -0700 [thread overview] Message-ID: <CA+55aFz4hWfT5c93rUWvN4OsYHjOSAjmNtoT7Rkjz7kYsaC7xg@mail.gmail.com> (raw) In-Reply-To: <4F6BA69F.1040707@openvz.org> On Thu, Mar 22, 2012 at 3:24 PM, Konstantin Khlebnikov <khlebnikov@openvz.org> wrote: > > # define __nocast __attribute__((nocast)) > > typedef long __nocast long_t; So the intention is that this really creates a *new* type. So "long_t" really is a different type from "long", but because __nocast is so weak, it happily casts to another integer type of the same size. But a pointer to it is different, the same way "int *" is different from "long *" even if "int" and "long" happen to have the same size. So I do think that the warning you quote is correct and expected: > 1.c:13:12: warning: incorrect type in argument 1 (different modifiers) > 1.c:13:12: expected int [nocast] [usertype] *x > 1.c:13:12: got int *<noident> > 1.c:13:12: warning: implicit cast to nocast type > > Is this ok? Yes. The thing about __nocast is that it's so *very* very easy to lose it. For example, do this: typedef long __nocast long_t; int main(long_t a) { return a; } and you get the (expected) warning. HOWEVER. Now do "return a+1" instead, and the warning goes away. Why? Because the expression ends up having just the type "long", because the "a" mixed happily with the "1" (that was cast from 'int' to 'long' by the normal C type rules). That is arguably a bug, but this kind of thing really wasn't what __nocast was designed for. The __nocast design ended up being too weak, though, and we hardly use it in the kernel. Linus
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From: Linus Torvalds <torvalds@linux-foundation.org> To: Konstantin Khlebnikov <khlebnikov@openvz.org> Cc: Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Minchan Kim <minchan@kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Ben Herrenschmidt <benh@kernel.crashing.org>, "linux@arm.linux.org.uk" <linux@arm.linux.org.uk> Subject: Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit Date: Thu, 22 Mar 2012 15:39:00 -0700 [thread overview] Message-ID: <CA+55aFz4hWfT5c93rUWvN4OsYHjOSAjmNtoT7Rkjz7kYsaC7xg@mail.gmail.com> (raw) In-Reply-To: <4F6BA69F.1040707@openvz.org> On Thu, Mar 22, 2012 at 3:24 PM, Konstantin Khlebnikov <khlebnikov@openvz.org> wrote: > > # define __nocast __attribute__((nocast)) > > typedef long __nocast long_t; So the intention is that this really creates a *new* type. So "long_t" really is a different type from "long", but because __nocast is so weak, it happily casts to another integer type of the same size. But a pointer to it is different, the same way "int *" is different from "long *" even if "int" and "long" happen to have the same size. So I do think that the warning you quote is correct and expected: > 1.c:13:12: warning: incorrect type in argument 1 (different modifiers) > 1.c:13:12: expected int [nocast] [usertype] *x > 1.c:13:12: got int *<noident> > 1.c:13:12: warning: implicit cast to nocast type > > Is this ok? Yes. The thing about __nocast is that it's so *very* very easy to lose it. For example, do this: typedef long __nocast long_t; int main(long_t a) { return a; } and you get the (expected) warning. HOWEVER. Now do "return a+1" instead, and the warning goes away. Why? Because the expression ends up having just the type "long", because the "a" mixed happily with the "1" (that was cast from 'int' to 'long' by the normal C type rules). That is arguably a bug, but this kind of thing really wasn't what __nocast was designed for. The __nocast design ended up being too weak, though, and we hardly use it in the kernel. Linus -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2012-03-22 22:39 UTC|newest] Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-03-21 6:56 [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 01/16] mm: introduce NR_VMA_FLAGS Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 02/16] mm: use vm_flags_t for vma flags Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 03/16] mm/shmem: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 04/16] mm/nommu: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 7:08 ` Greg Ungerer 2012-03-21 7:08 ` Greg Ungerer 2012-03-21 7:20 ` Konstantin Khlebnikov 2012-03-21 7:20 ` Konstantin Khlebnikov 2012-03-21 12:01 ` [PATCH v2 " Konstantin Khlebnikov 2012-03-21 12:01 ` Konstantin Khlebnikov 2012-03-23 6:47 ` Greg Ungerer 2012-03-23 6:47 ` Greg Ungerer 2012-03-21 6:56 ` [PATCH 05/16] mm/drivers: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 10:34 ` Laurent Pinchart 2012-03-21 10:34 ` Laurent Pinchart 2012-03-21 14:46 ` Greg Kroah-Hartman 2012-03-21 14:46 ` Greg Kroah-Hartman 2012-03-21 6:56 ` [PATCH 06/16] mm/x86: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:57 ` H. Peter Anvin 2012-03-21 6:57 ` H. Peter Anvin 2012-03-21 6:56 ` [PATCH 07/16] mm/arm: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-22 21:21 ` Andrew Morton 2012-03-22 21:21 ` Andrew Morton 2012-03-22 21:21 ` Andrew Morton 2012-03-21 6:56 ` [PATCH 08/16] mm/unicore32: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-27 3:38 ` Guan Xuetao 2012-03-27 3:38 ` Guan Xuetao 2012-03-27 5:58 ` Konstantin Khlebnikov 2012-03-27 5:58 ` Konstantin Khlebnikov 2012-03-27 7:50 ` Guan Xuetao 2012-03-27 7:50 ` Guan Xuetao 2012-03-21 6:56 ` [PATCH 09/16] mm/ia64: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 10/16] mm/powerpc: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:56 ` [PATCH 11/16] mm/s390: " Konstantin Khlebnikov 2012-03-21 6:56 ` Konstantin Khlebnikov 2012-03-21 6:57 ` [PATCH 12/16] mm/mips: " Konstantin Khlebnikov 2012-03-21 6:57 ` Konstantin Khlebnikov 2012-03-21 6:57 ` [PATCH 13/16] mm/parisc: " Konstantin Khlebnikov 2012-03-21 6:57 ` Konstantin Khlebnikov 2012-03-21 6:57 ` [PATCH 14/16] mm/score: " Konstantin Khlebnikov 2012-03-21 6:57 ` Konstantin Khlebnikov 2012-03-21 6:57 ` [PATCH 15/16] mm: cast vm_flags_t to u64 before printing Konstantin Khlebnikov 2012-03-21 6:57 ` Konstantin Khlebnikov 2012-03-21 6:57 ` [PATCH 16/16] mm: vm_flags_t strict type checking Konstantin Khlebnikov 2012-03-21 6:57 ` Konstantin Khlebnikov 2012-03-21 12:11 ` [PATCH v2 " Konstantin Khlebnikov 2012-03-21 12:11 ` Konstantin Khlebnikov 2012-03-21 10:06 ` [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit Minchan Kim 2012-03-21 10:06 ` Minchan Kim 2012-03-21 13:16 ` Konstantin Khlebnikov 2012-03-21 13:16 ` Konstantin Khlebnikov 2012-03-22 5:39 ` Minchan Kim 2012-03-22 5:39 ` Minchan Kim 2012-03-22 6:22 ` Benjamin Herrenschmidt 2012-03-22 6:22 ` Benjamin Herrenschmidt 2012-03-24 14:46 ` Konstantin Khlebnikov 2012-03-24 14:46 ` Konstantin Khlebnikov 2012-03-24 15:00 ` Konstantin Khlebnikov 2012-03-24 15:00 ` Konstantin Khlebnikov 2012-03-24 23:50 ` Benjamin Herrenschmidt 2012-03-24 23:50 ` Benjamin Herrenschmidt 2012-03-25 7:55 ` Konstantin Khlebnikov 2012-03-25 7:55 ` Konstantin Khlebnikov 2012-03-22 21:26 ` Andrew Morton 2012-03-22 21:26 ` Andrew Morton 2012-03-22 21:28 ` Al Viro 2012-03-22 21:28 ` Al Viro 2012-03-22 21:41 ` Andrew Morton 2012-03-22 21:41 ` Andrew Morton 2012-03-22 21:57 ` Al Viro 2012-03-22 21:57 ` Al Viro 2012-03-22 22:05 ` Konstantin Khlebnikov 2012-03-22 22:05 ` Konstantin Khlebnikov 2012-03-22 22:24 ` Konstantin Khlebnikov 2012-03-22 22:24 ` Konstantin Khlebnikov 2012-03-22 22:39 ` Linus Torvalds [this message] 2012-03-22 22:39 ` Linus Torvalds 2012-03-22 22:52 ` Konstantin Khlebnikov 2012-03-22 22:52 ` Konstantin Khlebnikov 2012-03-22 23:09 ` Andrew Morton 2012-03-22 23:09 ` Andrew Morton 2012-03-23 1:42 ` Al Viro 2012-03-23 1:42 ` Al Viro 2012-03-22 22:08 ` Linus Torvalds 2012-03-22 22:08 ` Linus Torvalds 2012-03-23 16:19 ` KOSAKI Motohiro 2012-03-23 16:19 ` KOSAKI Motohiro 2012-03-30 2:19 ` Al Viro 2012-03-30 2:19 ` Al Viro
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