On Tue, 2015-03-17 at 16:35 -0400, Steven Rostedt wrote:
> On Tue, 17 Mar 2015 12:35:43 -0400
> Steven Rostedt <[email protected]> wrote:
>
> > 3.10.70-rt75-rc2 stable review patch.
> > If anyone has any objections, please let me know.
> >
> > ------------------
> >
>
> Here's the missing change log for this revert. I'll go back and add it
> in:
>
>
> An issue arisen that if a rt_mutex (spin_lock converted to a mutex
> in PREEMPT_RT) is taken in hard interrupt context, it could cause
> a false deadlock detection and trigger a BUG_ON() from the return
> value of task_blocks_on_rt_mutex() in rt_spin_lock_slowlock().
>
> The problem is this:
>
> CPU0 CPU1
> ---- ----
> spin_lock(A)
> spin_lock(A)
> [ blocks, but spins as owner on
> CPU 0 is running ]
>
> <interrupt>
> spin_trylock(B)
> [ succeeds ]
>
> spin_lock(B)
> <blocks>
>
> Now the deadlock detection triggers and follows the locking:
>
> Task X (on CPU0) blocked on spinlock B owned by task Y on
> CPU1 (via the interrupt taking it with a try lock)
>
> The owner of B (Y) is blocked on spin_lock A (still spinning)
> A is owned by task X (self). DEADLOCK detected! BUG_ON triggered.
>
> This was caused by the code to try to not raise softirq unconditionally
> to allow NO_HZ_FULL to work. Unfortunately, reverting that patch causes
> NO_HZ_FULL to break again, but that's still better than triggering
> a BUG_ON().
(aw crap, let's go shopping)... so why is the one in timer.c ok?
-Mike
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