linux-kernel - Re: [PATCH v2 0/9] Remove spin_unlock

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Date:   Fri, 7 Jul 2017 12:33:49 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "manfred@...orfullife.com" <manfred@...orfullife.com>,
        "tj@...nel.org" <tj@...nel.org>, "arnd@...db.de" <arnd@...db.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()


* Peter Zijlstra <peterz@...radead.org> wrote:

> You missed the one in do_exit(), which I thought was the original one.

Indeed, it's raw_spin_unlock_wait() which my git grep pattern missed.

But it's not the original spin_unlock_wait(): the pi_lock and priority inheritance 
is a newfangled invention that Linus (rightfully) resisted for years.

The original spin_unlock_wait() was for the global scheduler_lock, long gone.

Here's the full history of the original spin_unlock_wait() usecase in the 
do_exit() path, for the historically interested:

 [1997/04] v2.1.36:

      the spin_unlock_wait() primitive gets introduced as part of release()

 [1998/08] v2.1.114:

      the release() usecase gets converted to an open coded spin_lock()+unlock() 
      poll loop over scheduler_lock

 [1999/05] v2.3.11pre3:

      open coded loop is changed over to poll p->has_cpu

 [1999/07] v2.3.12pre6:

      ->has_cpu loop poll loop is converted to a spin_lock()+unlock() 
      poll loop over runqueue_lock

 [2000/06] 2.4.0-test6pre4:

      combined open coded p->has_cpu poll loop is added back, in addition to the
      lock()+unlock() loop

 [2000/11] 2.4.0-test12pre4:

      lock+unlock loop is changed from scheduler_lock to task_lock

 [2001/11] v2.4.14.9:

       ->has_cpu gets renamed to ->cpus_runnable

 [2001/12] v2.5.1.10:

       poll loop is factored out from exit()'s release() function 
       to the scheduler's new wait_task_inactive() function

 ...

 [2017/07] v4.12:

      wait_task_inactive() is still alive and kicking. Its poll loop has
      increased in complexity, but it still does not use spin_unlock_wait()

So it was always a mess, and we relatively early flipped from the clever 
spin_unlock_wait() implementation to an open coded lock+unlock poll loop.

TL;DR: The original do_exit() usecase is gone, it does not use spin_unlock_wait(),
       since 1998.

Thanks,

	Ingo