On Wed, 19 Feb 2025 21:15:08 -0500
Waiman Long <llong@xxxxxxxxxx> wrote:
>
> On 2/19/25 8:41 PM, Steven Rostedt wrote:
> > On Wed, 19 Feb 2025 20:36:13 -0500
> > Waiman Long <llong@xxxxxxxxxx> wrote:
> >
> >
> >>>>>> this field, we don't need to take lock, though taking the wait_lock may
> >>>>>> still be needed to examine other information inside the mutex.
> >>> Do we need to take it just for accessing owner, which is in an atomic?
> >> Right. I forgot it is an atomic_long_t. In that case, no lock should be
> >> needed.
> > Now if we have a two fields to read:
> >
> > block_flags (for the type of lock) and blocked_on (for the lock)
> >
> > We need a way to synchronize the two. What happens if we read the type, and
> > the task wakes up and and then blocks on a different type of lock?
> >
> > Then the lock read from blocked_on could be a different type of lock than
> > what is expected.
>
> That is different from reading the owner. In this case, we need to use
> smp_rmb()/wmb() to sequence the read and write operations unless it is
> guaranteed that they are in the same cacheline. One possible way is as
> follows:
>
> Writer - setting them:
>
> WRITE_ONCE(lock)
> smp_wmb()
> WRITE_ONCE(type)
>
> Clearing them:
>
> WRITE_ONCE(type, 0)
> smp_wmb()
> WRITE_ONCE(lock, NULL)
>
> Reader:
>
> READ_ONCE(type)
> again:
> smp_rmb()
> READ_ONCE(lock)
> smp_rmb()
> if (READ_ONCE(type) != type)
> goto again
What about mutex-rwsem-mutex case?
mutex_lock(&lock1);
down_read(&lock2);
mutex_lock(&lock3);
The worst scenario is;
WRITE_ONCE(lock, &lock1)
smp_wmb()
WRITE_ONCE(type, MUTEX) READ_ONCE(type) -> MUTEX
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)
WRITE_ONCE(lock, &lock2) READ_ONCE(lock) -> &lock2
smp_wmb()
WRITE_ONCE(type, RWSEM)
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)
WRITE_ONCE(lock, &lock3)
smp_wmb()
WRITE_ONCE(type, MUTEX) READ_ONCE(type) -> MUTEX == MUTEX
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)
"OK, lock2 is a MUTEX!"
So unless stopping the blocker task, we can not ensure this works.
But unless decode the lock, we don't know the blocker task.
Maybe we can run the hung_task in stop_machine()?
(or introduce common_lock)
Thank you,
--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
