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Date: Tue, 20 Jan 2015 09:23:35 -0800
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Rusty Russell <rusty@...tcorp.com.au>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: module: fix module_refcount() return when running in a module
exit routine
On Tue, 2015-01-20 at 11:15 +1030, Rusty Russell wrote:
> James Bottomley <James.Bottomley@...senPartnership.com> writes:
> > On Mon, 2015-01-19 at 16:21 +1030, Rusty Russell wrote:
> >> Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> writes:
> >> > (2015/01/19 1:55), James Bottomley wrote:
> >> >> From: James Bottomley <JBottomley@...allels.com>
> >> >>
> >> >> After e513cc1 module: Remove stop_machine from module unloading,
> >> >> module_refcount() is returning (unsigned long)-1 when called from within
> >> >> a routine that runs in module_exit. This is confusing the scsi device
> >> >> put code which is coded to detect a module_refcount() of zero for
> >> >> running within a module exit routine and not try to do another
> >> >> module_put. The fix is to restore the original behaviour of
> >> >> module_refcount() and return zero if we're running inside an exit
> >> >> routine.
> >> >>
> >> >> Fixes: e513cc1c07e2ab93a4514eec9833e031df3e30bb
> >> >> Reported-by: Bart Van Assche <bvanassche@....org>
> >> >> Signed-off-by: James Bottomley <JBottomley@...allels.com>
> >> >>
> >> >
> >> > Yes, this should be fixed as you said, since it must return "unsigned long" value.
> >>
> >> But there are only three non-module callers:
> >>
> >> drivers/scsi/scsi.c:1012: if (module && module_refcount(module) != 0)
> >> drivers/staging/lustre/lustre/obdclass/lu_object.c:1359: LINVRNT(module_refcount(key->lct_owner) > 0);
> >> include/linux/module.h:447:unsigned long module_refcount(struct module *mod);
> >> kernel/debug/kdb/kdb_main.c:2026: kdb_printf("%4ld ", module_refcount(mod));
> >> kernel/module.c:775:unsigned long module_refcount(struct module *mod)
> >> kernel/module.c:779:EXPORT_SYMBOL(module_refcount);
> >> kernel/module.c:859: seq_printf(m, " %lu ", module_refcount(mod));
> >> kernel/module.c:911: return sprintf(buffer, "%lu\n", module_refcount(mk->mod));
> >>
> >> The first one I think should be eliminated, and the second one is simply
> >> an assertion before calling module_put() (which should probably be
> >> eliminated). The others are just printing information.
> >
> > If you really want to insist on module_reference() returning -1 when the
> > module is in it's exit phase, OK, but in that case, I think it should
> > return a signed value, not an unsigned one.
>
> Sure; I just didn't want to paper over the problem here. And I'm not
> sure we want to lose information, eg. in kgdb we're presumably looking
> at it because something went wrong...
>
> Thanks,
> Rusty.
>
> Subject: module: make module_refcount() a signed integer.
>
> James Bottomley points out that it will be -1 during unload. It's
> only used for diagnostics, so let's not hide that as it could be a
> clue as to what's gone wrong.
>
> Cc: Jason Wessel <jason.wessel@...driver.com>
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index ebfb0e153c6a..b653d7c0a05a 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -444,7 +444,7 @@ extern void __module_put_and_exit(struct module *mod, long code)
> #define module_put_and_exit(code) __module_put_and_exit(THIS_MODULE, code)
>
> #ifdef CONFIG_MODULE_UNLOAD
> -unsigned long module_refcount(struct module *mod);
> +int module_refcount(struct module *mod);
> void __symbol_put(const char *symbol);
> #define symbol_put(x) __symbol_put(VMLINUX_SYMBOL_STR(x))
> void symbol_put_addr(void *addr);
> diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
> index f191bddf64b8..7b40c5f07dce 100644
> --- a/kernel/debug/kdb/kdb_main.c
> +++ b/kernel/debug/kdb/kdb_main.c
> @@ -2023,7 +2023,7 @@ static int kdb_lsmod(int argc, const char **argv)
> kdb_printf("%-20s%8u 0x%p ", mod->name,
> mod->core_size, (void *)mod);
> #ifdef CONFIG_MODULE_UNLOAD
> - kdb_printf("%4ld ", module_refcount(mod));
> + kdb_printf("%4d ", module_refcount(mod));
> #endif
> if (mod->state == MODULE_STATE_GOING)
> kdb_printf(" (Unloading)");
> diff --git a/kernel/module.c b/kernel/module.c
> index 3965511ae133..2387c98347c1 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -772,9 +772,9 @@ static int try_stop_module(struct module *mod, int flags, int *forced)
> return 0;
> }
>
> -unsigned long module_refcount(struct module *mod)
> +int module_refcount(struct module *mod)
> {
> - return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE;
> + return atomic_read(&mod->refcnt) - MODULE_REF_BASE;
> }
> EXPORT_SYMBOL(module_refcount);
>
> @@ -856,7 +856,7 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod)
> struct module_use *use;
> int printed_something = 0;
>
> - seq_printf(m, " %lu ", module_refcount(mod));
> + seq_printf(m, " %i ", module_refcount(mod));
>
> /*
> * Always include a trailing , so userspace can differentiate
> @@ -908,7 +908,7 @@ EXPORT_SYMBOL_GPL(symbol_put_addr);
> static ssize_t show_refcnt(struct module_attribute *mattr,
> struct module_kobject *mk, char *buffer)
> {
> - return sprintf(buffer, "%lu\n", module_refcount(mk->mod));
> + return sprintf(buffer, "%i\n", module_refcount(mk->mod));
> }
Actually, I don't think this is enough. Some Australian once came up
with a guide to APIs, and lectured on it at length, one of which was
that the name should be the obvious use and it is unexpected that a
refcount would go negative. I think we could raise it from -6 on the
API scale to +3 if we add some documentation like below.
James
---
diff --git a/include/linux/module.h b/include/linux/module.h
index ebfb0e1..b653d7c 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -444,7 +444,7 @@ extern void __module_put_and_exit(struct module *mod, long code)
#define module_put_and_exit(code) __module_put_and_exit(THIS_MODULE, code)
#ifdef CONFIG_MODULE_UNLOAD
-unsigned long module_refcount(struct module *mod);
+int module_refcount(struct module *mod);
void __symbol_put(const char *symbol);
#define symbol_put(x) __symbol_put(VMLINUX_SYMBOL_STR(x))
void symbol_put_addr(void *addr);
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index f191bdd..7b40c5f 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -2023,7 +2023,7 @@ static int kdb_lsmod(int argc, const char **argv)
kdb_printf("%-20s%8u 0x%p ", mod->name,
mod->core_size, (void *)mod);
#ifdef CONFIG_MODULE_UNLOAD
- kdb_printf("%4ld ", module_refcount(mod));
+ kdb_printf("%4d ", module_refcount(mod));
#endif
if (mod->state == MODULE_STATE_GOING)
kdb_printf(" (Unloading)");
diff --git a/kernel/module.c b/kernel/module.c
index 3965511..63fc779 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -772,9 +772,18 @@ static int try_stop_module(struct module *mod, int flags, int *forced)
return 0;
}
-unsigned long module_refcount(struct module *mod)
+/**
+ * module_refcount - return the refcount or -1 if unloading
+ *
+ * @mod: the module we're checking
+ *
+ * Returns:
+ * -1 if the module is in the process of unloading
+ * otherwise the number of references in the kernel to the module
+ */
+int module_refcount(struct module *mod)
{
- return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE;
+ return atomic_read(&mod->refcnt) - MODULE_REF_BASE;
}
EXPORT_SYMBOL(module_refcount);
@@ -856,7 +865,7 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod)
struct module_use *use;
int printed_something = 0;
- seq_printf(m, " %lu ", module_refcount(mod));
+ seq_printf(m, " %i ", module_refcount(mod));
/*
* Always include a trailing , so userspace can differentiate
@@ -908,7 +917,7 @@ EXPORT_SYMBOL_GPL(symbol_put_addr);
static ssize_t show_refcnt(struct module_attribute *mattr,
struct module_kobject *mk, char *buffer)
{
- return sprintf(buffer, "%lu\n", module_refcount(mk->mod));
+ return sprintf(buffer, "%i\n", module_refcount(mk->mod));
}
static struct module_attribute modinfo_refcnt =
--
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