linux-kernel - Re: [kernel-locking] question about structure field initialization

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Date:   Fri, 12 May 2017 09:38:36 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [kernel-locking] question about structure field initialization

On Thu, May 11, 2017 at 03:00:02PM -0500, Gustavo A. R. Silva wrote:
> 
> Hello everybody,
> 
> While looking into Coverity ID 1402035 I ran into the following
> piece of code at kernel/locking/test-ww_mutex.c:197:
> 
> 197static int test_abba(bool resolve)
> 198{
> 199        struct test_abba abba;
> 200        struct ww_acquire_ctx ctx;
> 201        int err, ret;
> 202
> 203        ww_mutex_init(&abba.a_mutex, &ww_class);
> 204        ww_mutex_init(&abba.b_mutex, &ww_class);
> 205        INIT_WORK_ONSTACK(&abba.work, test_abba_work);
> 206        init_completion(&abba.a_ready);
> 207        init_completion(&abba.b_ready);
> 208        abba.resolve = resolve;
> 209
> 210        schedule_work(&abba.work);
> 211
> 212        ww_acquire_init(&ctx, &ww_class);
> 213        ww_mutex_lock(&abba.a_mutex, &ctx);
> 214
> 215        complete(&abba.a_ready);
> 216        wait_for_completion(&abba.b_ready);
> 217
> 218        err = ww_mutex_lock(&abba.b_mutex, &ctx);
> 219        if (resolve && err == -EDEADLK) {
> 220                ww_mutex_unlock(&abba.a_mutex);
> 221                ww_mutex_lock_slow(&abba.b_mutex, &ctx);
> 222                err = ww_mutex_lock(&abba.a_mutex, &ctx);
> 223        }
> 224
> 225        if (!err)
> 226                ww_mutex_unlock(&abba.b_mutex);
> 227        ww_mutex_unlock(&abba.a_mutex);
> 228        ww_acquire_fini(&ctx);
> 229
> 230        flush_work(&abba.work);
> 231        destroy_work_on_stack(&abba.work);
> 232
> 233        ret = 0;
> 234        if (resolve) {
> 235                if (err || abba.result) {
> 236                        pr_err("%s: failed to resolve ABBA
> deadlock, A err=%d, B err=%d\n",
> 237                               __func__, err, abba.result);
> 238                        ret = -EINVAL;
> 239                }
> 240        } else {
> 241                if (err != -EDEADLK && abba.result != -EDEADLK) {
> 242                        pr_err("%s: missed ABBA deadlock, A
> err=%d, B err=%d\n",
> 243                               __func__, err, abba.result);
> 244                        ret = -EINVAL;
> 245                }
> 246        }
> 247        return ret;
> 248}
> 
> The issue here is that apparently abba.result is being used at lines
> 235, 237 and 241 without previous initialization.
> 
> It seems to me that this is an issue, but I may be overlooking something.
> Can someone help me to spot where exactly abba.result is being
> initialized, if at all?

You are only looking at half the code. Though the schedule/flush it is
indirectly executing test_abba_work().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre