module: fix module_refcount() return when running in a module exit routine

[James Bottomley <James.Bottomley@HansenPartnership.com>]

From: James Bottomley <JBottomley@Parallels.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@acm.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

---

diff --git a/kernel/module.c b/kernel/module.c
index 3965511..c33a113 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -774,7 +774,12 @@ static int try_stop_module(struct module *mod, int flags, int *forced)
 
 unsigned long module_refcount(struct module *mod)
 {
-	return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE;
+	unsigned long ret = atomic_read(&mod->refcnt);
+
+	if (ret == 0)
+		/* ref is already zero (probably in module exit) */
+		return 0;
+	return ret - MODULE_REF_BASE;
 }
 EXPORT_SYMBOL(module_refcount);