Re: [PATCH 3.14.25-rt22 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!

From: Steven Rostedt
Date: Thu Feb 19 2015 - 23:53:07 EST

On Thu, 19 Feb 2015 18:31:05 -0700
Thavatchai Makphaibulchoke <tmac@xxxxxx> wrote:

> This patch fixes the problem that the ownership of a mutex acquired by an
> interrupt handler(IH) gets incorrectly attributed to the interrupted thread.


> This could result in an incorrect deadlock detection in function
> rt_mutex_adjust_prio_chain(), causing thread to be killed and possibly leading
> up to a system hang.

I highly doubt this is an incorrect deadlock that was detected. My
money is on a real deadlock that happened.

> Here is the approach taken: when calling from an interrupt handler, instead of
> attributing ownership to the interrupted task, use a reserved task_struct value
> to indicate that the owner is a interrupt handler. This approach avoids the
> incorrect deadlock detection.

How is this an incorrect deadlock? Please explain.

> This also includes changes in several function in rtmutex.c now that the lock's
> requester may be a interrupt handler, not a real task struct. This impacts
> the way how the lock is acquired and prioritized and decision whether to do
> the house keeping functions required for a real task struct.
> The reserved task_struct values for interrupt handler are
> current | 0x2
> where current is the task_struct value of the interrupted task.
> Since IH will both acquire and release the lock only during an interrupt
> handling, during which current is not changed, the reserved task_struct value
> for an IH should be distinct from another instances of IH on a different cpu.

The interrupt handler is a thread just like any other task. It's not
special. If there was a deadlock detected, it most likely means that a
deadlock exists.

-- Steve

> Kernel version 3.14.25 + patch-3.14.25-rt22
> Signed-off-by: T. Makphaibulchoke <tmac@xxxxxx>
> ---
> include/linux/spinlock_rt.h | 4 +
> kernel/locking/rtmutex-debug.c | 15 ++-
> kernel/locking/rtmutex.c | 212 ++++++++++++++++++++++++++++------------
> kernel/locking/rtmutex_common.h | 21 ++++
> kernel/timer.c | 4 +-
> 5 files changed, 188 insertions(+), 68 deletions(-)

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