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Date: Tue, 30 Jun 2020 09:28:44 -0700
From: Hydro Flask <hydroflask@yqxmail.com>
To: musl@lists.openwall.com
Cc: Markus Wichmann <nullplan@gmx.net>
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Message-ID: <275470aa6820d420339929a1fe409d89@yqxmail.com>
Subject: Re: [musl] Potential deadlock in pthread_kill()

On 2020-06-30 07:58, Markus Wichmann wrote:
> On Tue, Jun 30, 2020 at 05:26:46AM -0400, Rich Felker wrote:
>> On Mon, Jun 29, 2020 at 11:19:39PM -0700, Hydro Flask wrote:
>> >
>> > Just to be clear, this doesn't only occur when calling
>> > pthread_kill() and using pthread_self() as the target, it can be any
>> > target thread, as long as it's the same target thread is used in the
>> > signal handler and in the synchronous context.
>> 
>> How so? If the target is different, the rest of the pthread_kill,
>> including the unlock, will proceed concurrently with the signal
>> handler. However you may be able to construct mutual-signaling
>> deadlock cases.
>> 
> 
> Thread A calls pthread_kill(thread_c, ...). Thread B calls
> (concurrently) pthread_kill(thread_a, ...). Thread B's signal arrives
> while thread A holds the killlock. Signal handler calls
> pthread_kill(thread_c, ...).

You can trigger it much more simply than that. Doesn't require multiple 
threads:

1. pthread_kill(th)
2. LOCK(killlock)
3. <signal arrives after lock>
4. signal handler: pthread_kill(th)
5. signal handler: LOCK(killlock)

In the preceding example "th" can be any pthread_t value, the only 
requirement is that it's the same pthread_t in both the synchronous and 
signal handler context.