[PATCH v2 3/3] tracing: Show TID and flags for PI futex system call trace event

[Steven Rostedt]

From: Steven Rostedt <rostedt@xxxxxxxxxxx>

For the futex system call trace event for FUTEX_LOCK_PI and
FUTEX_UNLOCK_PI commands, show the TID from the value (which is usually in
hex) as well as translate the flags (DIED and WAITERS).

 pi_mutex_hammer-1098    [000] .....   121.876928: sys_futex(uaddr: 0x560f40cc8180 (0x450) tid: 1104, FUTEX_LOCK_PI|FUTEX_PRIVATE_FLAG, val: 0, timespec: 0x7f2f9d4b1e50 (0.000100000))
 pi_mutex_hammer-1128    [000] .....   121.877120: sys_futex(uaddr: 0x560f40cc8180 (0x8000042a) tid: 1066 (WAITERS), FUTEX_LOCK_PI|FUTEX_PRIVATE_FLAG, val: 0, timespec: 0x7f2f8e493e50 (0.000100000))
 pi_mutex_hammer-1106    [000] .....   121.877242: sys_futex(uaddr: 0x560f40cc8180 (0x80000452) tid: 1106 (WAITERS), FUTEX_UNLOCK_PI|FUTEX_PRIVATE_FLAG, val: 0)

This makes it easier to see the hand off of a mutex and who the owner was.

Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v1: https://lore.kernel.org/all/20260303174442.548b6524@xxxxxxxxxxxxxxxxxx/

- Updated to have the print processing in kernel/futex/syscall.c

 kernel/futex/syscalls.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index a46706d6bc6c..24d912d9b20d 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -211,6 +211,9 @@ void futex_print_syscall(struct seq_buf *s, int nr_args, unsigned long *args,
 	unsigned int op, cmd;
 	bool done = false;
 
+	op = args[1];
+	cmd = op & FUTEX_CMD_MASK;
+
 	for (int i = 0; !done && i < nr_args; i++) {
 
 		if (seq_buf_has_overflowed(s))
@@ -225,11 +228,30 @@ void futex_print_syscall(struct seq_buf *s, int nr_args, unsigned long *args,
 					seq_buf_printf(s, " (%u)", val);
 				else
 					seq_buf_printf(s, " (0x%x)", val);
+
+				switch(cmd) {
+				case FUTEX_LOCK_PI:
+				case FUTEX_UNLOCK_PI:
+					seq_buf_printf(s, " tid: %d",
+						       val & FUTEX_TID_MASK);
+
+					if (!(val & (FUTEX_OWNER_DIED|FUTEX_WAITERS)))
+						break;
+
+					seq_buf_puts(s, " (");
+					if (val & FUTEX_WAITERS)
+						seq_buf_puts(s, "WAITERS");
+					if (val & FUTEX_OWNER_DIED) {
+						if (op & FUTEX_WAITERS)
+							seq_buf_putc(s, '|');
+						seq_buf_puts(s, "DIED");
+					}
+					seq_buf_putc(s, ')');
+					break;
+				}
 			}
 			continue;
 		case 1:
-			op = args[i];
-			cmd = op & FUTEX_CMD_MASK;
 			if (cmd <= FUTEX_LOCK_PI2)
 				seq_buf_printf(s, ", %s", __futex_cmds[cmd]);
 			else
-- 
2.51.0