Sasha Levin wrote:

quoted

While it seems that 9p is the culprit, I have to point out that this
bug is easily reproducible, and it happens each time due to a
call_usermode_helper() call. Other than that 9p behaves perfectly and
I'd assume that I'd be seeing other things break besides
call_usermode_helper() related ones.

I think one of below two patches can catch the bug if this is a usermodehelper
related bug. Please try.

----- Patch 1 -----

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 01394b6..3e63319 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c

@@ -571,7 +571,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)

        * flag, for khelper thread is already waiting for the thread at
        * wait_for_completion() in do_fork().
        */
-       if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
+       if (WARN_ON(wait != UMH_NO_WAIT && current == kmod_thread_locker)) {
               retval = -EBUSY;
               goto out;
       }


----- Patch 2 -----

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 9efeae6..1350670 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h

@@ -48,10 +48,10 @@ static inline int request_module_nowait(const char *name, ...) { return -ENOSYS;

 struct cred;
 struct file;

-#define UMH_NO_WAIT    0       /* don't wait at all */
-#define UMH_WAIT_EXEC  1       /* wait for the exec, but not the process */
-#define UMH_WAIT_PROC  2       /* wait for the process to complete */
-#define UMH_KILLABLE   4       /* wait for EXEC/PROC killable */
+#define UMH_NO_WAIT    0x10    /* don't wait at all */
+#define UMH_WAIT_EXEC  0x11    /* wait for the exec, but not the process */
+#define UMH_WAIT_PROC  0x12    /* wait for the process to complete */
+#define UMH_KILLABLE   0x04    /* wait for EXEC/PROC killable */

 struct subprocess_info {
       struct work_struct work;

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 957a7aa..ecfd3d5 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c

@@ -483,6 +483,18 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)

       DECLARE_COMPLETION_ONSTACK(done);
       int retval = 0;

+       if (unlikely(wait == -1 || wait == 0 || wait == 1)) {
+               WARN(1, "Requesting for usermode helper with hardcoded wait "
+                    "flag. Change to use UMH_* symbols and recompile, or "
+                    "this request will fail on Linux 3.4.\n");
+               if (wait == -1)
+                       wait = UMH_NO_WAIT;
+               else if (wait == 0)
+                       wait = UMH_WAIT_EXEC;
+               else
+                       wait = UMH_WAIT_PROC;
+       }
+
       helper_lock();
       if (sub_info->path[0] == '\0')
               goto out;