Philippe Gerum wrote:
> Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>> Jan Kiszka wrote:
>>>> 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...
>>> See commit #3795, and change log entry from 2008-05-15. Unless I 
>>> misunderstood
>>> your description, this bug was fixed in 2.4.4.
>> Oh, fatally missed that fix.
>> Anyway, the patch looks a bit unclean to me. Either you are lacking
>> wwake = NULL in xnpod_suspend_thread, or the whole information encoded
>> in XNWAKEN can already be covered by wwake directly.
> Clearing wwake has to be done when returning from xnsynch_sleep_on, only when
> the code knows that ownership is eventually granted to the caller; making 
> such a
> decision in xnpod_suspend_thread() would be wrong.

What about


> The awake bit has been kept mainly because the nucleus commonly uses bitmasks 
> to
> get fast access to thread status & information. It's not mandatory to have 
> this
> one in, it's just conforming to the rest of the implementation.

I see, but redundancy come with some ugliness as well. And we add more
code to hot paths.


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