[lkml]   [2006]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: robust futex deadlock detection patch

On Fri, 6 Jan 2006, david singleton wrote:

> Here is a new patch that provides both futex deadlock detection and
> prevents ill-behaved and
> malicious apps from deadlocking the kernel through the robust futex
> interface.
> Deadlock detection is done 'up front' for both POSIX and robust
> pthread_mutexes. Non-recursive
> POSIX mutexes will hang if deadlocked, as defined by the POSIX spec.
> The wait channel they
> are hung on is 'futex_deadlock'. This wait channel makes it easy to
> spot that your POSIX app
> has deadlocked itself via the 'ps' command.
> Robust futexes will have -EDEADLK returned to them since there is no
> POSIX specification for
> robust mutexes, yet, and returning -EDEADLK is more in the spirit of
> robustness. Robust
> mutexes are cleaned up by the kernel after a thread dies and they also
> report to the app if
> it is deadlocking itself.
> Deadlock detection is something I have wanted to provide for both debug
> and production kernels
> for a while. It was previously available through DEBUG_DEADLOCKS. I
> needed to add the
> deadlock dection code for both production and debug kernels to prevent
> applications hanging
> the kernel.

I am a little bit confused when I read check_futex_deadlock():
It takes a parameter struct thread_info *ti and immediately do
struct task_struct *task = ti->task. Now we have the usual pair
(thread_info *ti, task_t *task) corresponding to the same process. Later
on in the function you do ti = lock_owner(lock), but do not update task.
Was this intented?

Anyway, I can't see that you have locked the necesary raw_spin_locks.
Forinstance lock_owner(lock) must be called with the lock->wait_lock taken
and task->blocked_on needs task->pi_lock locked.
To avoid deadlocks in all the deadlock detection you have to do the loop
something like

for(owner = current; owner; ) {
if(owner->task->blocked_on) {
lock = owner->task->blocked_on->lock;
owner2 = lock_owner(lock);
if(owner2) {
owner = owner2;
if(owner2==current) DEADLOCK


> David
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to
> More majordomo info at
> Please read the FAQ at

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-01-09 10:27    [W:0.044 / U:10.580 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site