2018-08-11 03:02:01

by Jia-Ju Bai

Subject: [PATCH V2] kernel: locking: rtmutex: Fix a possible sleep-in-atomic-context bug in rt_mutex_handle_deadlock()

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 the loop of schedule()
This is found by my static analysis tool (DSAC).

Signed-off-by: Jia-Ju Bai <[email protected]>
---
v2:
* Release the spinlock before the loop, instead of v1 that releases the
spinlock before schedule() and then acquires the spinlock again.
Thank Steven for good advice.
---
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..8f25a289abe8 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
@@ -1218,6 +1218,8 @@ static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
* Yell lowdly and stop the task right here.
*/
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();
@@ -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

2018-09-10 14:00:38

by Peter Zijlstra

Subject: Re: [PATCH V2] kernel: locking: rtmutex: Fix a possible sleep-in-atomic-context bug in rt_mutex_handle_deadlock()

On Sat, Aug 11, 2018 at 11:00:37AM +0800, Jia-Ju Bai wrote:

You forgot to Cc the person who wrote this code...

> 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..8f25a289abe8 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
> @@ -1218,6 +1218,8 @@ static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
> * Yell lowdly and stop the task right here.
> */
> 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();
> @@ -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);
> }

The patch is correct; but I can't find myself liking it very much. This
dinly little single use function is growing a lot of arguments.

The alternative is something like the below; not sure myself though.
Thomas?


diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2823d4163a37..a44d4034e232 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1204,18 +1204,10 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
return ret;
}

-static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
- struct rt_mutex_waiter *w)
+static void rt_mutex_handle_deadlock(struct rt_mutex_waiter *w)
{
/*
- * If the result is not -EDEADLOCK or the caller requested
- * deadlock detection, nothing to do here.
- */
- if (res != -EDEADLOCK || detect_deadlock)
- return;
-
- /*
- * Yell lowdly and stop the task right here.
+ * Yell loudly and stop the task right here.
*/
rt_mutex_print_deadlock(w);
while (1) {
@@ -1269,7 +1261,10 @@ 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);
+ if (chwalk == RT_MUTEX_MIN_CHAINWALK && ret == -EDEADLOCK) {
+ raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+ rt_mutex_handle_deadlock(&waiter);
+ }
}

/*