Re: softlockups in multi_cpu_stop

[Ming Lei]

On Sat, Mar 7, 2015 at 12:31 PM, Jason Low <jason.low2@xxxxxx> wrote:
> On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
>
> Just in case, here's the updated patch which addresses Linus's comments
> and with a changelog.
>
> Note: The changelog says that it fixes (locking/rwsem: Avoid deceiving
> lock spinners), though I still haven't seen full confirmation that it
> addresses all of the lockup reports.
>
> ------
> Subject: [PATCH] rwsem: Avoid spinning when owner is not running
>
> Fixes tip commmit b3fd4f03ca0b (locking/rwsem: Avoid deceiving lock spinners).
>
> When doing optimistic spinning in rwsem, threads should stop spinning when
> the lock owner is not running. While a thread is spinning on owner, if
> the owner reschedules, owner->on_cpu returns false and we stop spinning.
>
> However, commit b3fd4f03ca0b essentially caused the check to get ignored
> because when we break out of the spin loop due to !on_cpu, we continue
> spinning if sem->owner != NULL.
>
> This patch fixes this by making sure we stop spinning if the owner is not
> running. Furthermore, just like with mutexes, refactor the code such that
> we don't have separate checks for owner_running(). This makes it more
> straightforward in terms of why we exit the spin on owner loop and we
> would also avoid needing to "guess" why we broke out of the loop to make
> this more readable.
>
> Cc: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Signed-off-by: Jason Low <jason.low2@xxxxxx>

Reported-and-tested-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>

> ---
>  kernel/locking/rwsem-xadd.c |   31 +++++++++++--------------------
>  1 files changed, 11 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 06e2214..3417d01 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -324,32 +324,23 @@ done:
>         return ret;
>  }
>
> -static inline bool owner_running(struct rw_semaphore *sem,
> -                                struct task_struct *owner)
> -{
> -       if (sem->owner != owner)
> -               return false;
> -
> -       /*
> -        * Ensure we emit the owner->on_cpu, dereference _after_ checking
> -        * sem->owner still matches owner, if that fails, owner might
> -        * point to free()d memory, if it still matches, the rcu_read_lock()
> -        * ensures the memory stays valid.
> -        */
> -       barrier();
> -
> -       return owner->on_cpu;
> -}
> -
>  static noinline
>  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
>  {
>         long count;
>
>         rcu_read_lock();
> -       while (owner_running(sem, owner)) {
> -               /* abort spinning when need_resched */
> -               if (need_resched()) {
> +       while (sem->owner == owner) {
> +               /*
> +                * Ensure we emit the owner->on_cpu, dereference _after_
> +                * checking sem->owner still matches owner, if that fails,
> +                * owner might point to free()d memory, if it still matches,
> +                * the rcu_read_lock() ensures the memory stays valid.
> +                */
> +               barrier();
> +
> +               /* abort spinning when need_resched or owner is not running */
> +               if (!owner->on_cpu || need_resched()) {

BTW, could the need_resched() be handled in loop of
rwsem_optimistic_spin() directly? Then code may get
simplified a bit.


Thanks,
Ming Lei
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