On Thu, Nov 30, 2023 at 02:27:29PM +0100, Sebastian Andrzej Siewior wrote:
> It is claimed that srcu_read_lock_nmisafe() NMI-safe. However it
> triggers a lockdep if used from NMI because lockdep expects a deadlock
> since nothing disables NMIs while the lock is acquired.
>
> This is because commit f0f44752f5f61 ("rcu: Annotate SRCU's update-side
> lockdep dependencies") annotates synchronize_srcu() as a write lock
> usage. This helps to detect a deadlocks such as
> srcu_read_lock();
> synchronize_srcu();
> srcu_read_unlock();
>
> The side effect is that the lock srcu_struct now has a USED usage in normal
> contexts, so it conflicts with a USED_READ usage in NMI. But this shouldn't
> cause a real deadlock because the write lock usage from synchronize_srcu() is a
> fake one and only used for read/write deadlock detection.
>
> Use a try-lock annotation for srcu_read_lock_nmisafe() to avoid lockdep
> complains if used from NMI.
>
> Fixes: f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")
> Link: https://lore.kernel.org/r/20230927160231.XRCDDSK4@xxxxxxxxxxxxx
> Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Queued for v6.9 along with further review and testing, thank you both!
Thanx, Paul
> ---
> include/linux/rcupdate.h | 6 ++++++
> include/linux/srcu.h | 2 +-
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index f7206b2623c98..31d523c4e0893 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -301,6 +301,11 @@ static inline void rcu_lock_acquire(struct lockdep_map *map)
> lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
> }
>
> +static inline void rcu_try_lock_acquire(struct lockdep_map *map)
> +{
> + lock_acquire(map, 0, 1, 2, 0, NULL, _THIS_IP_);
> +}
> +
> static inline void rcu_lock_release(struct lockdep_map *map)
> {
> lock_release(map, _THIS_IP_);
> @@ -315,6 +320,7 @@ int rcu_read_lock_any_held(void);
> #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
>
> # define rcu_lock_acquire(a) do { } while (0)
> +# define rcu_try_lock_acquire(a) do { } while (0)
> # define rcu_lock_release(a) do { } while (0)
>
> static inline int rcu_read_lock_held(void)
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 127ef3b2e6073..236610e4a8fa5 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
>
> srcu_check_nmi_safety(ssp, true);
> retval = __srcu_read_lock_nmisafe(ssp);
> - rcu_lock_acquire(&ssp->dep_map);
> + rcu_try_lock_acquire(&ssp->dep_map);
> return retval;
> }
>
> --
> 2.43.0
