[Linux-kernel-mentees] [PATCH v2] callchain: Annotate RCU pointer with __rcu

Fixes following instances of sparse error
error: incompatible types in comparison expression
(different address spaces)
kernel/events/callchain.c:66:9: error: incompatible types in comparison
kernel/events/callchain.c:96:9: error: incompatible types in comparison
kernel/events/callchain.c:161:19: error: incompatible types in comparison

This introduces the following warning
kernel/events/callchain.c:65:17: warning: incorrect type in assignment
which is fixed as below

callchain_cpus_entries is annotated as an RCU pointer.
Hence rcu_dereference_protected or similar RCU API is
required to dereference the pointer.

Signed-off-by: Amol Grover <redacted>
---
v2:
- Squash both the commits into a single one.

 kernel/events/callchain.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index c2b41a263166..a672d02a1b3a 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c

@@ -32,7 +32,7 @@ static inline size_t perf_callchain_entry__sizeof(void)
 static DEFINE_PER_CPU(int, callchain_recursion[PERF_NR_CONTEXTS]);
 static atomic_t nr_callchain_events;
 static DEFINE_MUTEX(callchain_mutex);
-static struct callchain_cpus_entries *callchain_cpus_entries;
+static struct callchain_cpus_entries __rcu *callchain_cpus_entries;
 
 
 __weak void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,

@@ -62,7 +62,8 @@ static void release_callchain_buffers(void)
 {
 	struct callchain_cpus_entries *entries;
 
-	entries = callchain_cpus_entries;
+	entries = rcu_dereference_protected(callchain_cpus_entries,
+					    lockdep_is_held(&callchain_mutex));
 	RCU_INIT_POINTER(callchain_cpus_entries, NULL);
 	call_rcu(&entries->rcu_head, release_callchain_buffers_rcu);
 }

-- 
2.24.1

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