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SubjectRe: [PATCH] kernel: locking: rtmutex: Fix a possible sleep-in-atomic-context bug in rt_mutex_handle_deadlock()
On Sat, Aug 11, 2018 at 10:35:24AM +0800, Jia-Ju Bai wrote:
> The driver may sleep with holding a spinlock.
>
> The function call paths (from bottom to top) in Linux-4.16 are:
>
> [FUNC] schedule
> kernel/locking/rtmutex.c, 1223:
> schedule in rt_mutex_handle_deadlock
> kernel/locking/rtmutex.c, 1273:
> rt_mutex_handle_deadlock in rt_mutex_slowlock
> kernel/locking/rtmutex.c, 1249:
> _raw_spin_lock_irqsave in rt_mutex_slowlock
>
> To fix the bug, the spinlock is released before schedule() and then acquired again.
> This is found by my static analysis tool (DSAC).
>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
> kernel/locking/rtmutex.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 2823d4163a37..af03e162f812 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1205,7 +1205,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
> }
>
> static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
> - struct rt_mutex_waiter *w)
> + struct rt_mutex_waiter *w, struct rt_mutex *lock)
> {
> /*
> * If the result is not -EDEADLOCK or the caller requested
> @@ -1219,8 +1219,10 @@ static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
> */
> rt_mutex_print_deadlock(w);
> while (1) {
> + raw_spin_unlock_irq(&lock->wait_lock);
> set_current_state(TASK_INTERRUPTIBLE);
> schedule();
> + raw_spin_lock_irq(&lock->wait_lock);
> }

If you look at the code you will notice that it stops the task and never lets
it continue. Ever.

If we hit this path, it means we are in a deadlock scenario and will not make
any forward progress.

If anything, it should simply be:

rt_mutex_print_deadlock(w);
+ /* We're not going anywhere, release the wait_lock */
+ raw_spin_unlock_irq(&lock->wait_lock);
while (1) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
}

-- Steve


> }
>
> @@ -1269,7 +1271,7 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
> if (unlikely(ret)) {
> __set_current_state(TASK_RUNNING);
> remove_waiter(lock, &waiter);
> - rt_mutex_handle_deadlock(ret, chwalk, &waiter);
> + rt_mutex_handle_deadlock(ret, chwalk, &waiter, lock);
> }
>
> /*
> --
> 2.17.0

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