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Date:   Fri, 12 Jun 2020 18:33:45 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Xiaoming Ni <nixiaoming@huawei.com>
Cc:     paul@paul-moore.com, edumazet@google.com, paulmck@kernel.org,
        dhowells@redhat.com, keescook@chromium.org, shakeelb@google.com,
        jamorris@linux.microsoft.com, alex.huangjianhui@huawei.com,
        dylix.dailei@huawei.com, chenzefeng2@huawei.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] cred: Add WARN to detect wrong use of get/put_cred
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References: <1591957695-118312-1-git-send-email-nixiaoming@huawei.com>
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On Fri, Jun 12, 2020 at 06:28:15PM +0800, Xiaoming Ni wrote:
> Cred release and usage check code flow:
> 	1. put_cred()
> 		if (atomic_dec_and_test(&(cred)->usage))
> 			__put_cred(cred);
> 
> 	2. __put_cred()
> 		BUG_ON(atomic_read(&cred->usage) != 0);
> 		call_rcu(&cred->rcu, put_cred_rcu);
> 
> 	3. put_cred_rcu()
> 		if (atomic_read(&cred->usage) != 0)
> 			panic("CRED: put_cred_rcu() sees %p with usage %d\n",
> 			       cred, atomic_read(&cred->usage));
> 		kmem_cache_free(cred_jar, cred);
> 
> If panic is triggered on put_cred_rcu(), there are two possibilities
> 	1. Call get_cred() after __put_cred(), usage > 0
> 	2. Call put_cred() after __put_cred(), usage < 0
> Since put_cred_rcu is an asynchronous behavior, it is no longer the first
> scene when panic, there is no information about the murderer in the panic
> call stack...
> 
> So, add WARN() in get_cred()/put_cred(), and pray to catch the murderer
> at the first scene.

Why not not use refcount_t? It has all that goodness and more.