Messages in this thread Patch in this message | | | Date | Wed, 14 May 2014 20:03:26 -0000 | From | Thomas Gleixner <> | Subject | [patch 1/2] rtmutex: Fix deadlock detector for real |
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The current deadlock detection logic does not work reliably due to the following early exit path:
/* * Drop out, when the task has no waiters. Note, * top_waiter can be NULL, when we are in the deboosting * mode! */ if (top_waiter && (!task_has_pi_waiters(task) || top_waiter != task_top_pi_waiter(task))) goto out_unlock_pi;
So this not only exits when the task has no waiters, it also exits unconditionally when the current waiter is not the top priority waiter of the task.
So in a nested locking scenario, it might abort the lock chain walk and therefor miss a potential deadlock.
Simple fix: Continue the chain walk, when deadlock detection is enabled.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org --- kernel/locking/rtmutex.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-)
Index: linux-2.6/kernel/locking/rtmutex.c =================================================================== --- linux-2.6.orig/kernel/locking/rtmutex.c +++ linux-2.6/kernel/locking/rtmutex.c @@ -343,16 +343,24 @@ static int rt_mutex_adjust_prio_chain(st * top_waiter can be NULL, when we are in the deboosting * mode! */ - if (top_waiter && (!task_has_pi_waiters(task) || - top_waiter != task_top_pi_waiter(task))) - goto out_unlock_pi; + if (top_waiter) { + if (!task_has_pi_waiters(task)) + goto out_unlock_pi; + + if (top_waiter != task_top_pi_waiter(task)) { + if (!detect_deadlock) + goto out_unlock_pi; + } + } /* * When deadlock detection is off then we check, if further * priority adjustment is necessary. */ - if (!detect_deadlock && waiter->prio == task->prio) - goto out_unlock_pi; + if (waiter->prio == task->prio) { + if (!detect_deadlock) + goto out_unlock_pi; + } lock = waiter->lock; if (!raw_spin_trylock(&lock->wait_lock)) { @@ -527,6 +535,10 @@ static int task_blocks_on_rt_mutex(struc unsigned long flags; int chain_walk = 0, res; + /* Early deadlock detection */ + if (detect_deadlock && owner == task) + return -EDEADLK; + raw_spin_lock_irqsave(&task->pi_lock, flags); __rt_mutex_adjust_prio(task); waiter->task = task;
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