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

[Google]

On Tue, 10 Mar 2026 16:09:57 -0400
Steven Rostedt <rostedt@xxxxxxxxxx> wrote:

> 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>

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Thanks!


> ---
> 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
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>