[RFC PATCH] ftrace: Fix lost recursion records in ftrace_record_recursion()

[Andrea Parri <parri.andrea@gmail.com>]

cached_function is set to the current ip before the cmpxchg() that
claims a slot in recursed_functions[]. When that cmpxchg() loses a
race for the same slot, the code bumps index and retries via "goto
again", but the retry immediately matches its own cached_function
write and returns without ever reaching the bumped-index cmpxchg().

Concretely, for two writers racing on the same index:

  CPU 0 (ip = A)                    CPU 1 (ip = B)
  --------------                    --------------
                                     cmpxchg(&recursed_functions[index].ip,
                                             0, B);      // succeeds
  cached_function = A;
  cmpxchg(&recursed_functions[index].ip,
          0, A);
    // fails, old == B
  index++;
  goto again;
  if (A == cached_function)         // true: matches A's own store
          return;                   // A is dropped, not retried

Once dropped this way, cached_function stays set to A, so every later
recursion of A also hits the fast path and returns before reaching the
retry, until some unrelated ip overwrites the cache.

Only set cached_function once the record is confirmed present, either
because a concurrent writer already added it or because this writer
just claimed the slot. The retry path leaves it untouched, so it no
longer matches against a value it just wrote for itself.

Fixes: 773c16705058e ("ftrace: Add recording of functions that caused recursion")
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
Can't claim to fully understand the logic behind ftrace_record_recursion().
Notably, its smp_mb__after_atomic() calls and the lack of barrier comments
both look suspicious to my LKMM-trained eyes.  ;)  Hence the RFC.
---
 kernel/trace/trace_recursion_record.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/trace_recursion_record.c b/kernel/trace/trace_recursion_record.c
index bac4bc844ccd8..f42089c30f53c 100644
--- a/kernel/trace/trace_recursion_record.c
+++ b/kernel/trace/trace_recursion_record.c
@@ -17,8 +17,8 @@ static struct recursed_functions recursed_functions[CONFIG_FTRACE_RECORD_RECURSI
 static atomic_t nr_records;
 
 /*
- * Cache the last found function. Yes, updates to this is racey, but
- * so is memory cache ;-)
+ * Cache the last function confirmed present in recursed_functions[].
+ * Updates to this are racy, but this is only a best-effort cache.
  */
 static unsigned long cached_function;
 
@@ -67,24 +67,27 @@ void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip)
 		}
 	}
 
-	cached_function = ip;
-
 	/*
 	 * We only want to add a function if it hasn't been added before.
-	 * Add to the current location before incrementing the count.
-	 * If it fails to add, then increment the index (save in i)
-	 * and try again.
+	 * Claim the current slot before incrementing the count. If the slot
+	 * is occupied by another function, advance to the next slot and retry.
+	 *
+	 * Do not update cached_function until ip is known to be present;
+	 * otherwise the retry would match its own cache update.
 	 */
 	old = cmpxchg(&recursed_functions[index].ip, 0, ip);
 	if (old != 0) {
 		/* Did something else already added this for us? */
-		if (old == ip)
+		if (old == ip) {
+			cached_function = ip;
 			return;
+		}
 		/* Try the next location (use i for the next index) */
 		index++;
 		goto again;
 	}
 
+	cached_function = ip;
 	recursed_functions[index].parent_ip = parent_ip;
 
 	/*
-- 
2.53.0