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.
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/[email protected]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
---
This is a repost of
https://lore.kernel.org/r/[email protected]
Based on the discussion there I *think* this is preferred over the NMI
check in lock_acquire().
But then PeterZ also pointed out that he has a problem with
f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")
because trace_.*_rcuidle machinery. This looks okay because the _rcuidle
part is using SRCU and the rcu_dereference_raw() tracepoint_func is
using RCU + SRCU in its free part.
include/linux/rcupdate.h | 6 ++++++
include/linux/srcu.h | 2 +-
2 files changed, 7 insertions(+), 1 deletion(-)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -301,6 +301,11 @@ static inline void rcu_lock_acquire(stru
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)
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe
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;
}
On Tue, Nov 21, 2023 at 01:33:15PM +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.
>
Thanks for reposting!
I would add a paragraph here explaining why the commit is culprit:
This is because commit f0f44752f5f61 ("rcu: Annotate SRCU's update-side
lockdep dependencies") annotates synchronize_srcu() as a write lock
usage (so that srcu_read_lock(); synchronize_srcu() deadlock can be
found), 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/[email protected]
> Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
> ---
>
> This is a repost of
> https://lore.kernel.org/r/[email protected]
>
> Based on the discussion there I *think* this is preferred over the NMI
> check in lock_acquire().
> But then PeterZ also pointed out that he has a problem with
> f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")
>
> because trace_.*_rcuidle machinery. This looks okay because the _rcuidle
> part is using SRCU and the rcu_dereference_raw() tracepoint_func is
> using RCU + SRCU in its free part.
>
Yeah, I think we don't have more problems (famous last words).
Reviewed-by: Boqun Feng <[email protected]>
Regards,
Boqun
> include/linux/rcupdate.h | 6 ++++++
> include/linux/srcu.h | 2 +-
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -301,6 +301,11 @@ static inline void rcu_lock_acquire(stru
> 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)
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe
>
> 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;
> }
>
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/[email protected]
Reviewed-by: Boqun Feng <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
---
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
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/[email protected]
> Reviewed-by: Boqun Feng <[email protected]>
> Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
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