Messages in this thread | | | Date | Wed, 22 Mar 2023 16:15:06 +0200 | Subject | Re: [PATCH] perf/core: Fix the same task check in perf_event_set_output | From | Adrian Hunter <> |
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On 22/03/23 15:42, Peter Zijlstra wrote: > On Wed, Mar 22, 2023 at 12:59:28PM +0200, Adrian Hunter wrote: >> On 11/07/22 21:07, kan.liang@linux.intel.com wrote: >>> From: Kan Liang <kan.liang@linux.intel.com> >>> >>> With the --per-thread option, perf record errors out when sampling with >>> a hardware event and a software event as below. >>> >>> $ perf record -e cycles,dummy --per-thread ls >>> failed to mmap with 22 (Invalid argument) >>> >>> The same task is sampled with the two events. The IOC_OUTPUT is invoked >>> to share the mmap memory of the task between the events. In the >>> perf_event_set_output(), the event->ctx is used to check whether the >>> two events are attached to the same task. However, a hardware event and >>> a software event are from different task context. The check always >>> fails. >>> >>> The task struct is stored in the event->hw.target for each per-thread >>> event. It can be used to determine whether two events are attached to >>> the same task. >>> >>> The patch can also fix another issue reported months ago. >>> https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/ >>> The event->ctx is not ready when the perf_event_set_output() is invoked >>> in the perf_event_open(), while the event->hw.target has been assigned >>> at the moment. >>> >>> The problem should be a long time issue since commit c3f00c70276d >>> ("perf: Separate find_get_context() from event initialization"). The >>> event->hw.target doesn't exist at that time. Here, the patch which >>> introduces the event->hw.target is used by the Fixes tag. >>> >>> The problem should still exists between the broken patch and the >>> event->hw.target patch. This patch does not intend to fix that case. >>> >>> Fixes: 50f16a8bf9d7 ("perf: Remove type specific target pointers") >>> Reviewed-by: Zhengjun Xing <zhengjun.xing@linux.intel.com> >>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com> >> >> Did this slip through the cracks, or is there more complexity >> to this case than just sharing the rb? > > Both; I very much missed it, but looking at it now, I'm not at all sure > it is correct prior to the whole context rewrite we did recently. > > So after the rewrite every cpu/task only has a single > perf_event_context, and your change below is actually an equivalence. > > But prior to that a task could have multiple contexts. Now they got > co-scheduled most of the times and it will probably work, but I'm not > entirely sure. > > So how about we change the Fixes tag to something like: > > Fixes: c3f00c70276d ("perf: Separate find_get_context() from event initialization") # >= v6.2 > > And anybody that wants to back-port this further gets to either do the > full audit and/or keep the pieces. > > Hmm?
Seems reasonable to me. Kan?
> >>> --- >>> kernel/events/core.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/events/core.c b/kernel/events/core.c >>> index b4d62210c3e5..22df79d3f19d 100644 >>> --- a/kernel/events/core.c >>> +++ b/kernel/events/core.c >>> @@ -12080,7 +12080,7 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) >>> /* >>> * If its not a per-cpu rb, it must be the same task. >>> */ >>> - if (output_event->cpu == -1 && output_event->ctx != event->ctx) >>> + if (output_event->cpu == -1 && output_event->hw.target != event->hw.target) >>> goto out; >>> >>> /* >>
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