[Xenomai-core] [BUG] Lock stealing is borken

[Jan Kiszka <jan.kiszka@domain.hid>]

Hi,

bad news, everyone :(. According to the result of some lengthy debug
session with a customer and several ad-hoc lttng instrumentations, we
have a fatal bug in the nucleus' implementation of the lock stealing
algorithm. Consider this scenario:

1. Thread A acquires Mutex X successfully, ie. it leaves the (in this
   case) rt_mutex_acquire service, and its XNWAKEN flag is therefore
   cleared.

2. Thread A blocks on some further Mutex Y (in our case it was a
   semaphore, but that doesn't matter).

3. Thread B signals the availability of Mutex Y to Thread A, thus it
   also set XNWAKEN in Thread A. But Thread A is not yet scheduled on
   its CPU.

4. Thread C tries to acquire Mutex X, finds it assigned to Thread A, but
   also notices that the XNWAKEN flag of Thread A is set. Thus it steals
   the mutex although Thread A already entered the critical section -
   and hell breaks loose...

Looks like the XNWAKEN flag is misplaced in the owner's thread flags.
Can we safely move it into the synch object?

Jan

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