tracing/user_events: Document user_event_mm one-shot list usage

During 6.4 development it became clear that the one-shot list used by
the user_event_mm's next field was confusing to others. It is not clear
how this list is protected or what the next field usage is for unless
you are familiar with the code.

Add comments into the user_event_mm struct indicating lock requirement
and usage. Also document how and why this approach was used via comments
in both user_event_enabler_update() and user_event_mm_get_all() and the
rules to properly use it.

Link: https://lkml.kernel.org/r/20230519230741.669-5-beaub@linux.microsoft.com
Link: https://lore.kernel.org/linux-trace-kernel/CAHk-=wicngggxVpbnrYHjRTwGE0WYscPRM+L2HO2BF8ia1EXgQ@mail.gmail.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 22 insertions, 1 deletions

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 238c7a0615fa..dbb14705d0d3 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -451,12 +451,25 @@ static bool user_event_enabler_exists(struct user_event_mm *mm,
 static void user_event_enabler_update(struct user_event *user)
 {
 	struct user_event_enabler *enabler;
-	struct user_event_mm *mm = user_event_mm_get_all(user);
 	struct user_event_mm *next;
+	struct user_event_mm *mm;
 	int attempt;
 
 	lockdep_assert_held(&event_mutex);
 
+	/*
+	 * We need to build a one-shot list of all the mms that have an
+	 * enabler for the user_event passed in. This list is only valid
+	 * while holding the event_mutex. The only reason for this is due
+	 * to the global mm list being RCU protected and we use methods
+	 * which can wait (mmap_read_lock and pin_user_pages_remote).
+	 *
+	 * NOTE: user_event_mm_get_all() increments the ref count of each
+	 * mm that is added to the list to prevent removal timing windows.
+	 * We must always put each mm after they are used, which may wait.
+	 */
+	mm = user_event_mm_get_all(user);
+
 	while (mm) {
 		next = mm->next;
 		mmap_read_lock(mm->mm);
@@ -516,6 +529,14 @@ static struct user_event_mm *user_event_mm_get_all(struct user_event *user)
 	struct user_event_mm *mm;
 
 	/*
+	 * We use the mm->next field to build a one-shot list from the global
+	 * RCU protected list. To build this list the event_mutex must be held.
+	 * This lets us build a list without requiring allocs that could fail
+	 * when user based events are most wanted for diagnostics.
+	 */
+	lockdep_assert_held(&event_mutex);
+
+	/*
 	 * We do not want to block fork/exec while enablements are being
 	 * updated, so we use RCU to walk the current tasks that have used
 	 * user_events ABI for 1 or more events. Each enabler found in each