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Date: Tue, 18 Jan 2011 10:47:17 +0100
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: Re: [Xenomai-help] segfault sharing mutex from kernel space to user
	space
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To: Jeff Weber <jwamsc@domain.hid>
Cc: xenomai@xenomai.org

On 2011-01-17 21:15, Jeff Weber wrote:
> I get a segfault when attempting to rt_mutex_acquire a mutex created in
> kernel space.  I've reduced the issue to the following sample code.
> Help finding my mistake is appreciated.
> 
> TIA,
> Jeff
> 
> 
> Kernel space Code:
> #include <linux/module.h>
> #include <linux/init.h>
> #include <native/mutex.h>
> #include "testAPI.h"  /* defines MTXNAME */
> 
> #define MODNAME  "XenoTest"
> 
> static RT_MUTEX sMtx;
> 
> static int __init mymodule_init(void)
> {
>     int status;
> 
>     status = rt_mutex_create(&sMtx, MTXNAME);
>     if (status) {
>     printk ("rt_mutex_create: %d\n", status);
>         return 1;
>     }
> 
>     printk ("loaded module %s\n", MODNAME);
>     return 0;
> }
> 
> static void __exit mymodule_exit(void)
> {
>     rt_mutex_delete(&sMtx);
> 
>     printk ("unloaded module %s\n", MODNAME);
>     return;
> }
> 
> module_init(mymodule_init);
> module_exit(mymodule_exit);
> 
> MODULE_LICENSE("GPL");
> 
> 
> 
> User space Code:
> #include <stdio.h>
> #include <sys/mman.h>
> #include <native/mutex.h>
> #include <native/task.h>
> 
> #include "testAPI.h"    /* defines MTXNAME */
> 
> #define PRIO    0
> #define MODE    0
> 
> int main(void)
> {
>     RT_MUTEX mtx;
>     RT_TASK  tsk;
>     RT_MUTEX_INFO info;
>     int status;
> 
>     mlockall(MCL_CURRENT|MCL_FUTURE);
> 
>     status = rt_task_shadow(&tsk, NULL, PRIO, MODE);
>     if (status) {
>         fprintf(stderr, "rt_task_shadow: %d\n", status);
>         return 1;
>     }
> 
>     status = rt_mutex_bind(&mtx, MTXNAME, TM_INFINITE);
>     if (status) {
>         fprintf(stderr, "rt_mutex_bind: %d\n", status);
>         return 1;
>     }
> 
>     status = rt_mutex_inquire(&mtx, &info);
>     if (status) {
>         fprintf(stderr, "rt_mutex_inquire: %d\n", status);
>         return 1;
>     }
> 
>     status = rt_mutex_acquire(&mtx, TM_INFINITE); /* SEGFAULT HERE! */
>     if (status) {
>         fprintf(stderr, "rt_mutex_acquire: %d\n", status);
>         return 1;
>     }
> 
>     status = rt_mutex_release(&mtx);
>     if (status) {
>         fprintf(stderr, "rt_mutex_release: %d\n", status);
>         return 1;
>     }
> 
>     printf("test success\n");   // back to primary mode
>     return 0;
> }
> 
> my kernel
> 
> backtrace:
> Program terminated with signal 11, Segmentation fault.
> #0  0xb770077a in xnarch_atomic_cmpxchg (v=0xb777ac00, old=0, newval=21)
>     at ../../../src/include/asm/xenomai/atomic.h:95
> 95              __asm__ __volatile__(LOCK_PREFIX "cmpxchgl %1,%2"
> (gdb) bt full
> #0  0xb770077a in xnarch_atomic_cmpxchg (v=0xb777ac00, old=0, newval=21)
>     at ../../../src/include/asm/xenomai/atomic.h:95
>         ptr = 0xb777ac00
>         prev = 4294967295
> #1  0xb7700815 in xnsynch_fast_acquire (fastlock=0xb777ac00, new_ownerh=21)
>     at ../../../include/nucleus/synch.h:52
>         lock_state = 3077595124
> #2  0xb7700c3a in rt_mutex_acquire_inner (mutex=0xbfecd690, timeout=0,
>     mode=XN_RELATIVE) at mutex.c:83
>         err = 134513420
>         cur = 21
> #3  0xb7700e01 in rt_mutex_acquire (mutex=0xbfecd690, timeout=0) at
> mutex.c:129
> No locals.
> #4  0x0804884a in main () at uspace.c:38
>         mtx = {opaque = 19, fastlock = 0xb777ac00, lockcnt = 0}
>         tsk = {opaque = 21, opaque2 = 3075921616}
>         info = {locked = 0, nwaiters = 0,
>           name = "TestMtx\000\000\000\060\000@domain.hid%", '\000'
> <repeats 12 times>,
>           owner =
> "\000\000\000\000\364\036\331\336\020\037\331\336\365Pd\340\005\005UU\000\037\331\336\000\000\000\000\023\000\000"}
>         status = 0
> 
> my config:
> arch: x86
> linux: 2.6.35.10
> xenomai: 2.5.5.2
> 
> BTW: I did a checkout of git tag v2.5.5.2, and XENO_VERSION_STRING is
> "2.5.5.1"
> 

A) In-kernel use of the Xenomai skins is deprecated, and mixing user and
kernel space use won't make it easier for you to overcome this in your
system.

B) If you actually depend on a shared mutex (I would really recommend to
revalidate that need), you must create it in user space so that it gains
a user space compatible fastlock. Otherwise, the mutex will only be
initialized for in-kernel use, and binding to it from user space will
make the latter fail.

I guess we should catch this case more gracefully as long as we support
in-kernel mutexes...

Jan

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