Re: [PATCH v3 2/4] srcu: Add srcu_read_lock_fast_notrace() and srcu_read_unlock_fast_notrace()
From: Mathieu Desnoyers
Date: Wed Jul 23 2025 - 11:24:00 EST
On 2025-07-22 18:34, Paul E. McKenney wrote:
On Tue, Jul 22, 2025 at 06:11:00PM -0400, Joel Fernandes wrote:
On Mon, Jul 21, 2025 at 09:24:31AM -0700, Paul E. McKenney wrote:
This commit adds no-trace variants of the srcu_read_lock_fast() and
srcu_read_unlock_fast() functions for tracing use.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
include/linux/srcu.h | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 478c73d067f7d..7a692bf8f99b9 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -282,6 +282,20 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
return retval;
}
+/*
+ * Used by tracing, cannot be traced and cannot call lockdep.
+ * See srcu_read_lock_fast() for more information.
+ */
+static inline struct srcu_ctr __percpu *srcu_read_lock_fast_notrace(struct srcu_struct *ssp)
+ __acquires(ssp)
Should these also be marked with 'notrace' attribute?
I am not sure what the precedent is, I do see a few examples of 'notrace' and
'inline' in the same function signature though.
Heh!!!
There are six instance of static-inline notrace functions, and eight
instances of static-inline non-notrace functions whose names contain
"_notrace", not counting the srcu_read_lock_fast_notrace() and
srcu_read_unlock_fast() functions currently under review.
My guess is that I should add "notrace" to handle the possible case
where the compiler declines to inline this function. I will do this
on the next rebase unless I hear otherwise.
Steven, Mathieu, thoughts?
AFAIR, the always_inline gcc attribute takes care of making sure the
notrace is not needed in addition.
So I suspect that kernel APIs need to abide by the following rules
to be usable by instrumentation code:
- if it's a function call, have a notrace attribute.
- if it's an inline (which the compiler may decide to implement as a
function call), have a notrace attribute.
- if it's an __always_inline, then there is no way the compiler can
possibly make it a function call, so the notrace would be useless
there.
So you may want to choose for either:
- inline notrace, or
- __always_inline
Depending on how much freedom you want to grant the compiler with
respect to its inlining practices.
Thanks,
Mathieu
Thanx, Paul
Other than that one nit:
Reviewed-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
thanks,
- Joel
+{
+ struct srcu_ctr __percpu *retval;
+
+ srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
+ retval = __srcu_read_lock_fast(ssp);
+ return retval;
+}
+
/**
* srcu_down_read_fast - register a new reader for an SRCU-protected structure.
* @ssp: srcu_struct in which to register the new reader.
@@ -394,6 +408,17 @@ static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ct
RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
}
+/*
+ * Used by tracing, cannot be traced and cannot call lockdep.
+ * See srcu_read_unlock_fast() for more information.
+ */
+static inline void srcu_read_unlock_fast_notrace(struct srcu_struct *ssp,
+ struct srcu_ctr __percpu *scp) __releases(ssp)
+{
+ srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
+ __srcu_read_unlock_fast(ssp, scp);
+}
+
/**
* srcu_up_read_fast - unregister a old reader from an SRCU-protected structure.
* @ssp: srcu_struct in which to unregister the old reader.
--
2.40.1
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com