I'm not sure if those patches are connected, so I am posting my findings
for anyone.
ABOUT NOUVEAU AND WW_MUTEX:
The nouveau driver intensely uses the ww_mutex. According the
ww-mutex-design.txt documentation, the ww_mutex makes the following
steps when try to lock a resource:
1) It calls the ww_mutex_lock (or ww_mutex_lock_interruptible) function
for normal lock acquisition.
2) If it returns -EDEADLK, the wounded task must drop all already
acquired locks and call the ww_mutex_lock_slow (or
ww_mutex_lock_slow_interruptible) function.
In the nouveau driver, those steps happen in the validate_init function
(drivers/gpu/drm/nouveau_gem.c) when it calls the functions
ttm_bo_reserve (step 1) and ttm_bo_reserve_slowpath (step 2):
validate_init(...)
{
...
ret = ttm_bo_reserve(&nvbo->bo, true, false, true, &op->ticket);
if (ret) {
validate_fini_no_ticket(op, NULL);
if (unlikely(ret == -EDEADLK)) {
ret = ttm_bo_reserve_slowpath(&nvbo->bo, true,
&op->ticket);
...
}
THE PROBLEM:
When I run the PREEMPT_RT in my computer, it freezes as soon I start
some big graphical application (e.g. Firefox or Chrome). The following
execution path happens until reaching an infinite loop:
a) nouveau_gem_ioctl_pushbuf()
b) nouveau_gem_pushbuf_validate()
c) validate_init()
d) ttm_bo_reserve()
e) ttm_bo_reserve_nolru()
f) ww_mutex_lock_interruptible()
g) __ww_mutex_lock_interruptible()
h) rt_mutex_slowlock()
i) rt_mutex_handle_deadlock() -> infinite loop
The infinite loop that I mentioned above is the while(1){...} inside the
rt_mutex_handle_deadlock function:
static void rt_mutex_handle_deadlock(int res, int detect_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.
*/
rt_mutex_print_deadlock(w);
while (1) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
}
}
CONCLUSION:
My conclusion was the detect_deadlock should be enabled to the
rt_mutex_handle_deadlock function returns -EDEADLK and hence the
ww_mutex_lock_interruptible returns -EDEADLK too. BTW, since I have
applied my patch, I've not had anymore freezing issue.
PS: I am a kernelnewbie so it is possible that I am writing some BS here.
On 01/25/2015 06:55 PM, Paul Gortmaker wrote:
On Tue, Jan 20, 2015 at 3:02 PM, Gustavo Bittencourt <gbitten@xxxxxxxxx> wrote:
The functions ww_mutex_lock_interruptible and ww_mutex_lock should return -EDEADLK when faced with
a deadlock. To do so, the paramenter detect_deadlock in rt_mutex_slowlock must be TRUE.
This patch corrects potential deadlocks when running PREEMPT_RT with nouveau driver.
Kernel version: 3.14.25-rt22
You might want to have a look at this:
http://www.spinics.net/lists/linux-rt-users/msg12259.html
as it also impacts what the deadlock detection flag does.
Paul.
--
Signed-off-by: Gustavo Bittencourt <gbitten@xxxxxxxxx>
---
kernel/locking/rtmutex.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 6c40660..3f6ef91 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1965,7 +1965,7 @@ __ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ww_c
might_sleep();
mutex_acquire(&lock->base.dep_map, 0, 0, _RET_IP_);
- ret = rt_mutex_slowlock(&lock->base.lock, TASK_INTERRUPTIBLE, NULL, 0, ww_ctx);
+ ret = rt_mutex_slowlock(&lock->base.lock, TASK_INTERRUPTIBLE, NULL, 1, ww_ctx);
if (ret)
mutex_release(&lock->base.dep_map, 1, _RET_IP_);
else if (!ret && ww_ctx->acquired > 1)
@@ -1984,7 +1984,7 @@ __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ww_ctx)
mutex_acquire_nest(&lock->base.dep_map, 0, 0, &ww_ctx->dep_map,
_RET_IP_);
- ret = rt_mutex_slowlock(&lock->base.lock, TASK_UNINTERRUPTIBLE, NULL, 0, ww_ctx);
+ ret = rt_mutex_slowlock(&lock->base.lock, TASK_UNINTERRUPTIBLE, NULL, 1, ww_ctx);
if (ret)
mutex_release(&lock->base.dep_map, 1, _RET_IP_);
else if (!ret && ww_ctx->acquired > 1)
--
1.9.1
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