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Subject: Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock()
 like a trylock
To:     Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org
Cc:     linux-tip-commits@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        x86@kernel.org
References: <20210316153119.13802-4-longman@redhat.com>
 <161598470197.398.8903908266426306140.tip-bot2@tip-bot2>
 <YFIASRkXowQWgj2s@hirez.programming.kicks-ass.net>
 <YFIEo8IVQ/Mm9jUE@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <e1bcd7fb-3a40-f207-ee19-d276c8b8bb75@redhat.com>
Date:   Wed, 17 Mar 2021 10:03:07 -0400
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On 3/17/21 9:31 AM, Peter Zijlstra wrote:
> On Wed, Mar 17, 2021 at 02:12:41PM +0100, Peter Zijlstra wrote:
>> On Wed, Mar 17, 2021 at 12:38:21PM -0000, tip-bot2 for Waiman Long wrote:
>>> +	/*
>>> +	 * Treat as trylock for ww_mutex.
>>> +	 */
>>> +	mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, nest_lock, ip);
>> I'm confused... why isn't nest_lock working here?
>>
>> For ww_mutex, we're supposed to have ctx->dep_map as a nest_lock, and
>> all lock acquisitions under a nest lock should be fine. Afaict the above
>> is just plain wrong.
> To clarify:
>
> 	mutex_lock(&A);			ww_mutex_lock(&B, ctx);
> 	ww_mutex_lock(&B, ctx);		mutex_lock(&A);
>
> should still very much be a deadlock, but your 'fix' makes it not report
> that.
>
> Only order within the ww_ctx can be ignored, and that's exactly what
> nest_lock should be doing.
>
I will take a deeper look into why that is the case.

Cheers,
Longman