linux-kernel - Re: [PATCH v2 2/2] perf lock: Don't free "lock_seq_stat" if read

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Date:   Mon, 2 Nov 2020 17:56:26 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] perf lock: Don't free "lock_seq_stat" if
 read_count isn't zero

On Wed, Oct 21, 2020 at 08:39:48AM +0800, Leo Yan wrote:
> When execute command "perf lock report", it hits failure and outputs log
> as follows:
> 
>   perf: builtin-lock.c:623: report_lock_release_event: Assertion `!(seq->read_count < 0)' failed.
>   Aborted
> 
> This is an imbalance issue.  The locking sequence structure
> "lock_seq_stat" contains the reader counter and it is used to check if
> the locking sequence is balance or not between acquiring and releasing.
> 
> If the tool wrongly frees "lock_seq_stat" when "read_count" isn't zero,
> the "read_count" will be reset to zero when allocate a new structure at
> the next time; thus it causes the wrong counting for reader and finally
> results in imbalance issue.
> 
> To fix this issue, if detects "read_count" is not zero (means still
> have read user in the locking sequence), goto the "end" tag to skip
> freeing structure "lock_seq_stat".
> 
> Fixes: e4cef1f65061 ("perf lock: Fix state machine to recognize lock sequence")
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  tools/perf/builtin-lock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> index 5cecc1ad78e1..a2f1e53f37a7 100644
> --- a/tools/perf/builtin-lock.c
> +++ b/tools/perf/builtin-lock.c
> @@ -621,7 +621,7 @@ static int report_lock_release_event(struct evsel *evsel,
>  	case SEQ_STATE_READ_ACQUIRED:
>  		seq->read_count--;
>  		BUG_ON(seq->read_count < 0);
> -		if (!seq->read_count) {
> +		if (seq->read_count) {
>  			ls->nr_release++;

it seems ok, but I fail to see what's nr_release for
the point is just to skip the removal of seq right?

jirka

>  			goto end;
>  		}
> -- 
> 2.17.1
>