linux-kernel - Re: [PATCH RT 2/4] Revert "timers: do not raise softirq unconditionally"

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Message-ID: <20150317163551.3093b6c2@gandalf.local.home>
Date:	Tue, 17 Mar 2015 16:35:51 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Kacur <jkacur@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH RT 2/4] Revert "timers: do not raise softirq
 unconditionally"

On Tue, 17 Mar 2015 12:35:43 -0400
Steven Rostedt <rostedt@...dmis.org> 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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