linux-kernel - Re: Lock problems in linux/tools/perf/util/dso.c

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Date:   Wed, 29 Dec 2021 11:08:14 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ryan Cai <ycaibb@...il.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Lock problems in linux/tools/perf/util/dso.c

Hello,

On Wed, Dec 22, 2021 at 3:33 AM Ryan Cai <ycaibb@...il.com> wrote:
>
> Hi, I found a potential lock problem in dso_data_get_fd. Because the inconsistent branch conditions of pthread_mutex_lock(&dso__data_open_lock) and pthread_mutex_unlock(&dso__data_open_lock), it is possible that the lock dso__data_open_lock is not released in the dso_data_get_fd. Also, I have a question on why the branch condition of pthread_mutex_lock(&dso__data_open_lock) is <0. I think that the branch condition should be !=0, because pthread_mutex_lock would return 0 when succeeding. Looking forward to further discussion. One this bug is confirmed, I can send a patch.

Please fix your mail client to wrap around 80 characters.

1. dso__data_get_fd() should be paired with dso__data_put_fd()
   when it returns non-negative.  It'd unlock the mutex.

2. I've checked the man page and it seems you're right.
   It just says that it'd return an error number or zero.

   https://linux.die.net/man/3/pthread_mutex_lock

Thanks,
Namhyung


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> https://github.com/torvalds/linux/blob/e851dfae4371d3c751f1e18e8eb5eba993de1467/tools/perf/util/dso.c#L708-L722
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> Best,
>
> Ryan
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>