Re: [PATCH 1/1] UML: fix missing non-blocking I/O, now DEBUG_SHIRQworks

From: Eduard-Gabriel Munteanu
Date: Mon Jun 11 2007 - 18:39:41 EST

*This message was transferred with a trial version of CommuniGate(r) Pro*
Jeff Dike wrote:
*This message was transferred with a trial version of CommuniGate(r) Pro*
*This message was transferred with a trial version of CommuniGate(r) Pro*
On Mon, Jun 11, 2007 at 10:39:56PM +0300, Eduard-Gabriel Munteanu wrote:
No offense, but this is an ugly hack.

I'm not going to defend it too much, but the alternatives don't seem any
better to me.

What if sizeof(int) != sizeof(long)?

Doesn't matter - the casting will preserve the value. Of greater
concern is the relationship between sizeof(void *) and sizeof(long).
That's not guaranteed by the standard, but is by gcc.

The cast isn't done right. Doing "fd = (long) dev_id;" doesn't help, since you pass fd to mconsole_get_request() as is. And mconsole_get_request() expects an integer:
int mconsole_get_request(int fd, struct mc_request *req)

This will generate at least a warning on arches where sizeof(int) != sizeof(long). And as you say, then there is the problem with void pointers and longs.

And by the way, AFAIK, GCC has the habit of breaking compatibility with some userspace apps when a new GCC major version is released. And, AFAIK, they're moving towards standards, so relying on GCC's "guarantees" may backfire.

You're calling glibc functions
with that fd as a parameter. On some arches, compiling will issue
warnings or simply fail.

Which ones?

An example is sparc64:
quote from

38 #define BITS_PER_LONG 64
40 #ifndef __ASSEMBLY__
42 typedef __signed__ char s8;
43 typedef unsigned char u8;
45 typedef __signed__ short s16;
46 typedef unsigned short u16;
48 typedef __signed__ int s32;
49 typedef unsigned int u32;
51 typedef __signed__ long s64;
52 typedef unsigned long u64

An alternative would be to use kmalloc instead of a global static variable. Do you like this one more?

No, that trades the global variable for a new point of failure.

The main reason I like the current casting better than your global
(not by a lot) is that globals have to be audited for SMP safety. So,
I don't want any globals which don't need to be. This implies a
local, and to minimize the machinery associated with that (kmalloc, or
passing a pointer and synchronizing to avoid returning too soon), just
passing the descriptor in the pointer and accepting the casting needed
to get it through the compiler.


Really, a kmalloc() isn't such a big deal, it only happens once and we're not in interrupt context. One the other hand, ensuring safety and portability on other arches is something that needs to be taken care of.

Of course, you could also cast to int when calling mconsole_get_request(), but IMO that doesn't look nicer.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at