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Date: Sat, 2 Apr 2016 18:30:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Darren Hart <darren@dvhart.com>, Ingo Molnar <mingo@kernel.org>,
        Michael Kerrisk <mtk.manpages@googlemail.com>,
        Davidlohr Bueso <dave@stgolabs.net>, Chris Mason <clm@fb.com>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Torvald Riegel <triegel@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: [RFC patch 7/7] [PATCH] glibc: nptl: Add support for attached
 pthread_mutexes
Message-ID: <20160402163059.GS3448@twins.programming.kicks-ass.net>
References: <20160402095108.894519835@linutronix.de>
 <20160402110036.173191836@linutronix.de>
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On Sat, Apr 02, 2016 at 11:09:20AM -0000, Thomas Gleixner wrote:
> pthread_mutexes on Linux are based on the futex mechanism. The standard futex
> mechanism in the Linux kernel uses a global hash to store transient
> state. Collisions on that hash can lead to performance degradation and on
> real-time enabled kernels even to priority inversions.
> 
> To guarantee futexes without collisions on the global kernel hash, the kernel
> provides a mechanism to attach to a futex. This creates futex private state
> which avoids hash collisions and on NUMA systems also cross node memory
> access.
> 
> To utilize this mechanism each thread has to attach to the futex before any
> other operations on that futex which involve kernel interaction.
> 
> At pthread_mutex_init() the pthread_mutex attribute needs to be initialized
> for attached mode via:
> 
>     pthread_mutexattr_setattached_np(&attr, 1);
> 
> All threads which are using the mutex - including the one which called
> pthread_mutex_init() - must invoke
> 
>     pthread_mutex_attach_np(&mutex);
> 
> before any other pthread_mutex related operations.
> 
> Example:
>         pthread_mutexattr_t attr;
> 	pthread_mutex_t lock;
> 
>         pthread_mutexattr_init(&attr);
>         pthread_mutexattr_setattached_np(&attr, 1);
>         pthread_mutex_init(&lock, &attr);
> 
>         pthread_mutex_attach_np(&lock);
> 
> 	pthread_mutex_lock(&lock);
> 
> In ptrace this should look like this:
> 
> 	futex(<addr>, 0x280 /* FUTEX_??? */, 1, NULL <unfinished ...>
> 
> 	0x280: 	 FUTEX_ATTACHED | FUTEX_PRIVATE | FUTEX_WAIT
> 
> To undo the attachment each involved thread needs to call
> 
>     pthread_mutex_detach_np(&mutex);
>         
> When the last user detaches the kernel state is destroyed.

So I was fully expecting pthread_mutex_{at,de}tach_np() to not exist and
be internal to pthread_mutex_{init,destroy}().

Is there a reason this is not so?