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.

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


-- 
Philippe.

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