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SubjectRe: [RFC] Is an alternative module interface needed/possible?
In message <> you write:
> > failure: if the object is still busy, we just return -EBUSY. This is
> > simple, but this doesn't work for modules, since during module exit you
> > can't fail anymore.

Of course, they never could fail module exit. You could use
can_unload() which was only protected by the BKL, hence so rarely
used: grep 2.4.20 to see all four users.

Having a separate can_unload function means a window between asking
can_unload() and doing the actual unload. Doing it in the function
itself (ie. cleanup returns an int, or a separate "prepare_to_unload"
kind of function) means you have to deal with the "I started
deregistering my stuff, but then had to re-register them" which leaves
a race where the module is partially unavailable (ie. the spurious
failure problem).

Both these problems are soluble, but they're not trivial.

> That's a modules API problem. And yes, I think modules should
> eventually be able to say that they're busy.

Take a look at the current ip_conntrack code. It keeps its own
reference count and spins until it hits zero in unload, because
otherwise it would never be unloadable (it attaches a callback to each
packet, so logically it would bump its refcount there). In 2.5 it can
use try_module_get() and be unloaded with rmmod --wait in this worst
case (I'm slack, patching is on my todo list).

Security modules have the same issues, and Greg Kroah-Hartmann was
telling me USB has it as well.

You can see why I considered deprecating module unloading: it's a hard
problem, and fixing it in general probably means massive changes.
Which I'm happy to leave to someone else.

> By the way, a loong time ago, in the modules thread, I suggested
> a "decrement_module_count_and_return" function [1]. Such a
> construct would be useful in this specific case.

Stephen and I even implemented one, for x86 (see below). But
implementing the matching "enter and inc" case remains problematic, so
I dropped that idea.

Anyway, good luck!
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

Name: module_put_return primitive for x86
Author: Rusty Russell
Status: Experimental
Depends: Module/forceunload.patch.gz

D: This patch implements module_put_return() for x86, which allows a
D: module to control its own reference counts in some cases. A module
D: must never use module_put() on itself, since this may result in the
D: module being removable immediately: this is the alternative, which
D: atomically decrements the count and returns.
D: Each architecture will need to implement this for preempt.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .13134-linux-2.5.38/arch/i386/kernel/module.c .13134-linux-2.5.38.updated/arch/i386/kernel/module.c
--- .13134-linux-2.5.38/arch/i386/kernel/module.c 2002-09-25 02:02:28.000000000 +1000
+++ .13134-linux-2.5.38.updated/arch/i386/kernel/module.c 2002-09-25 02:02:56.000000000 +1000
@@ -28,6 +28,9 @@
#define DEBUGP(fmt , ...)

+/* For the magic module return. */
+struct module_percpu module_percpu[NR_CPUS];
static void *alloc_and_zero(unsigned long size)
void *ret;
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .13134-linux-2.5.38/arch/i386/lib/module.S .13134-linux-2.5.38.updated/arch/i386/lib/module.S
--- .13134-linux-2.5.38/arch/i386/lib/module.S 1970-01-01 10:00:00.000000000 +1000
+++ .13134-linux-2.5.38.updated/arch/i386/lib/module.S 2002-09-25 02:02:56.000000000 +1000
@@ -0,0 +1,33 @@
+/* Icky, icky, return trampoline for dying modules. Entered with
+ interrupts off. (C) 2002 Stephen Rothwell, IBM Corporation. */
+#include <asm/thread_info.h>
+.align 4
+.globl __magic_module_return
+ /* Save one working variable */
+ pushl %eax
+ /* Get CPU number from current. */
+ movl TI_CPU(%eax), %eax
+ /* Push module_percpu[cpu].flags on the stack */
+ pushl module_percpu(%eax)
+ /* Put module_percpu[cpu].returnaddr into %eax */
+ movl module_percpu+4(%eax), %eax
+ /* Push returnaddr and restore eax */
+ xchgl %eax, 4(%esp)
+ /* Restore interrupts */
+ popf
+ /* Go home */
+ ret
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .13134-linux-2.5.38/include/asm-i386/module.h .13134-linux-2.5.38.updated/include/asm-i386/module.h
--- .13134-linux-2.5.38/include/asm-i386/module.h 2002-09-25 02:02:28.000000000 +1000
+++ .13134-linux-2.5.38.updated/include/asm-i386/module.h 2002-09-25 02:02:56.000000000 +1000
@@ -8,4 +8,33 @@ struct mod_arch_specific
#define Elf_Shdr Elf32_Shdr
#define Elf_Sym Elf32_Sym
#define Elf_Ehdr Elf32_Ehdr
+/* Non-preemtible decrement the refcount and return. */
+#define module_put_return(firstarg , ...) \
+do { \
+ unsigned int cpu; \
+ unsigned long flags; \
+ \
+ local_irq_save(flags); \
+ if (unlikely(module_put(THIS_MODULE)) { \
+ module_percpu[cpu].flags = flags; \
+ module_percpu[cpu].returnaddr = ((void **)&(firstarg))[-1]; \
+ ((void **)&(firstarg))[-1] = __magic_module_return; \
+ } else \
+ local_irq_restore(flags); \
+ return __VA_ARGS__; \
+} while(0)
+/* FIXME: Use per-cpu vars --RR */
+struct module_percpu
+ unsigned long flags;
+ void *returnaddr;
+extern struct module_percpu module_percpu[NR_CPUS];
+/* Restore flags and return to caller. */
+extern void __magic_module_return(void);
#endif /* _ASM_I386_MODULE_H */
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