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Date:   Mon, 2 Mar 2020 13:28:29 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Jann Horn <jannh@google.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Alexey Dobriyan <adobriyan@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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Subject: Re: [PATCHv2] exec: Fix a deadlock in ptrace
Message-ID: <20200302122828.GA9769@redhat.com>
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On 03/01, Bernd Edlinger wrote:
>
> This fixes a deadlock in the tracer when tracing a multi-threaded
> application that calls execve while more than one thread are running.

Heh. Yes, known problem. See my attempt to fix it:
https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/

> @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>  	struct mm_struct *mm;
>  	int err;
>  
> -	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
> +	err =  mutex_lock_killable(&task->signal->cred_change_mutex);

So if I understand correctly your patch doesn't fix other problems
with debugger waiting for cred_guard_mutex.

I too do not think this can justify the new mutex in signal_struct...

Oleg.