On Tue, 17 Mar 2015 12:35:43 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> 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().
-- Steve
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