Tomas Kalibera wrote:
 > 
 > Hi,
 > 
 > I'm getting kernel crashes with my native skin user-space Xenomai 
 > application. It looks like the crash happens after clone/fork. I'm using 
 > kernel 2.6.24.3, SMP, RT_PREEMPT (settings like  2.6.22-14-rt from 
 > Ubuntu 7.10). Xenomai 2.4.2.
 > 
 > The thread causing the crash is a Xenomai task, running most of the time 
 > in the Linux domain. The application is very huge, getting a short 
 > example leading to the bug is unfortunatelly not realistic.
 > 
 > The crash happens when running on real hardware (x86_64 with 32 bit 
 > kernel and applications).  The system is unusable after it happens, can 
 > only be rebooted, the dump is from serial console.
 > In VMWare on another x86_64 machine, it does not crash.
 > 
 > Anyone getting a similar error ? Any ideas where to look for the problem ?

Looking at the kernel code, it seems that only one page may be mapped at
a time with kmap_atomic using KM_USER0. So what probably happens is that
for other invocations of cow_user_page than the one taking place in
fork, a lock of some kind prevents concurrent invocation of
cow_user_page. In our use of cow_user_page, we probably do not hold that
lock. I look at the code, I see that copy_pte_range holds a spinlock,
which should disable preemption on a classical kernel. But who knows
what happens with RT_PREEMPT enabled...

-- 


                                            Gilles.

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