[LU-15689] libcfs_debug_mb is not set correctly if module has not been initialized

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0
• Severity: 3
• Assignee: Chris Horn
• Reporter: Chris Horn

Description

If libcfs_debug_mb parameter is specified to insmod (i.e. set before module is initialized) then it does not get initialized correctly. This appears to be a regression from:

commit 8b78a3ffb5220330f41b4fa8576a05c4e017cfb1
Author: Mr NeilBrown <[e-mail]>
Date:   Sun Mar 8 21:35:57 2020 -0400

   LU-9859 libcfs: always range-check libcfs_debug_mb setting.

diff --git a/libcfs/libcfs/debug.c b/libcfs/libcfs/debug.c
index 99518ab116..e5bff2e18a 100644
--- a/libcfs/libcfs/debug.c
+++ b/libcfs/libcfs/debug.c
@@ -65,21 +65,15 @@ static int libcfs_param_debug_mb_set(const char *val,
        if (rc < 0)
                return rc;

-/*
- * RHEL6 does not support any kind of locking so we have to provide
- * our own
- */
-       if (!*((unsigned int *)kp->arg)) {
-               *((unsigned int *)kp->arg) = num;
-               return 0;
-       }
+       num = cfs_trace_set_debug_mb(num);

-       rc = cfs_trace_set_debug_mb(num);
-
-       if (!rc)
-               *((unsigned int *)kp->arg) = cfs_trace_get_debug_mb();
+       *((unsigned int *)kp->arg) = num;
+       num = cfs_trace_get_debug_mb();
+       if (num)
+               /* This value is more precise */
+               *((unsigned int *)kp->arg) = num;

-       return rc;
+       return 0;
 }

I think the idea is that cfs_trace_get_debug_mb() would only return non-zero if the module had been initialized, so it's return value should be used instead of the return val from cfs_trace_set_debug_mb(). However, cfs_trace_get_debug_mb() never returns 0:

int cfs_trace_get_debug_mb(void)
{
...
        return (total_pages >> (20 - PAGE_SHIFT)) + 1;
}

We can see it will always return at least 1.