[DTrace-devel] [PATCH 07/12] Fix drop counter handling for local-only updates

[Eugene Loh]

I'm not familiar with how drops are handled, but...
Reviewed-by: Eugene Loh <eugene.loh at oracle.com>

A 2006 copyright year on a "new" file looks weird.  Maybe it's right.  
I'm no lawyer.  Just weird.

On 1/5/24 00:29, Kris Van Hees via DTrace-devel wrote:
> When at the end of a work loop, the drop counters are updates for local

"are updates"
->
"are updated"

> updates only (i.e. when tracing is not stopped yet), an invalid drop
> count could be reported if a drop occured in the BEGIN probe.  The cause
> is that the code assumed that both copies of the status data contained
> valid data.  But when a drop occurs in the BEGIN probe, only the first
> copy will have non-zero values.  Since local-only updates assume that
> the two copies are identical except for a possible change due to the
> local update, and any earlier status update would already have reported
> any differences, it is safe to just copy all data prior to performing
> the local updates.
>
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   libdtrace/dt_work.c                           |  4 ++-
>   .../drops/drp.DTRACEDROP_DYNAMIC.local_only.d | 30 +++++++++++++++++++
>   .../drops/drp.DTRACEDROP_DYNAMIC.local_only.r |  6 ++++
>   3 files changed, 39 insertions(+), 1 deletion(-)
>   create mode 100644 test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.d
>   create mode 100644 test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.r
>
> diff --git a/libdtrace/dt_work.c b/libdtrace/dt_work.c
> index fe88a48e..6cac93ea 100644
> --- a/libdtrace/dt_work.c
> +++ b/libdtrace/dt_work.c
> @@ -1,6 +1,6 @@
>   /*
>    * Oracle Linux DTrace.
> - * Copyright (c) 2006, 2023, Oracle and/or its affiliates. All rights reserved.
> + * Copyright (c) 2006, 2024, Oracle and/or its affiliates. All rights reserved.
>    * Licensed under the Universal Permissive License v 1.0 as shown at
>    * http://oss.oracle.com/licenses/upl.
>    */
> @@ -271,6 +271,8 @@ dtrace_work(dtrace_hdl_t *dtp, FILE *fp, dtrace_consume_probe_f *pfunc,
>   	 * than once).
>   	 */
>   	if (!dtp->dt_stopped) {
> +		gen = dtp->dt_statusgen;
> +		dtp->dt_status[gen] = dtp->dt_status[gen ^ 1];
>   		dt_add_local_status(dtp);
>   		gen = dtp->dt_statusgen ^ 1;
>   	} else {
> diff --git a/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.d b/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.d
> new file mode 100644
> index 00000000..2ebcf150
> --- /dev/null
> +++ b/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.d
> @@ -0,0 +1,30 @@
> +/*
> + * Oracle Linux DTrace.
> + * Copyright (c) 2006, 2024, Oracle and/or its affiliates. All rights reserved.
> + * Licensed under the Universal Permissive License v 1.0 as shown at
> + * http://oss.oracle.com/licenses/upl.
> + */
> +
> +/*
> + * Ensure that when (at the end of a work loop) a local-only update is done, we
> + * do not end up reporting (unsigned long long)-1.
> + *
> + * If this bug exists, output will look like:
> + *	dtrace: [DTRACEDROP_DYNAMIC] 1 dynamic variable drop
> + *	dtrace: [DTRACEDROP_DYNAMIC] 18446744073709551615 dynamic variable drops
> + */
> +
> +#pragma D option dynvarsize=15
> +
> +BEGIN
> +{
> +	self->a = 1;
> +	self->b = 2;
> +	self->c = 3;
> +	self->d = 4;
> +}
> +
> +tick-1s
> +{
> +	exit(0);
> +}
> diff --git a/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.r b/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.r
> new file mode 100644
> index 00000000..8af0a5f5
> --- /dev/null
> +++ b/test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.r
> @@ -0,0 +1,6 @@
> +                   FUNCTION:NAME
> +                        :tick-1s
> +
> +-- @@stderr --
> +dtrace: script 'test/unittest/drops/drp.DTRACEDROP_DYNAMIC.local_only.d' matched 2 probes
> +dtrace: [DTRACEDROP_DYNAMIC] 1 dynamic variable drop