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Re: [PATCH v2 0/9] Remove spin_unlock_wait()

From Ingo Molnar <mingo@kernel.org>
Newsgroups linux.kernel
Subject Re: [PATCH v2 0/9] Remove spin_unlock_wait()
Date 2017-07-07 11:40 +0200
Message-ID <u0xTX-24L-9@gated-at.bofh.it> (permalink)
References <tXRvP-663-1@gated-at.bofh.it> <u023M-4zO-3@gated-at.bofh.it> <u0fNo-5Im-19@gated-at.bofh.it> <u0hvQ-7HB-11@gated-at.bofh.it> <u0wuR-17W-5@gated-at.bofh.it>
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > It might even be that this is the defined semantics of spin_unlock_wait().
> > 
> > As is, spin_unlock_wait() is somewhat ill defined. IIRC it grew from an 
> > optimization by Oleg and subsequently got used elsewhere. And it being the 
> > subtle bugger it is, there were bugs.
> 
> I believe the historical, original spin_unlock_wait() came from early SMP 
> optimizations of the networking code - and then spread elsewhere, step by step. 
> All but one of the networking uses went away since then - so I don't think there's 
> any original usecase left.

No - the original usecase was task teardown: I still remembered that but didn't 
find the commit - but it's there in very old Linux kernel patches, done by DaveM 
originally in v2.1.36 (!):

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -136,6 +136,12 @@ void release(struct task_struct * p)
        }
        for (i=1 ; i<NR_TASKS ; i++)
                if (task[i] == p) {
+#ifdef __SMP__
+                       /* FIXME! Cheesy, but kills the window... -DaveM */
+                       while(p->processor != NO_PROC_ID)
+                               barrier();
+                       spin_unlock_wait(&scheduler_lock);
+#endif


Other code learned to use spin_unlock_wait(): the original version of 
[hard]irq_enter() was the second user, net_family_read_lock() was the third user, 
followed by more uses in networking. All but one of those are not present in the 
current upstream kernel anymore.

This task-teardown FIXME was fixed in v2.1.114 (was replaced by an open coded poll 
loop), but the spin_unlock_wait() primitive remained.

The rest is history! ;-)

Thanks,

	Ingo

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Thread

[PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
  [PATCH v2 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
  RE: [PATCH v2 0/9] Remove spin_unlock_wait() David Laight <David.Laight@ACULAB.COM> - 2017-07-06 16:20 +0200
    Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 17:30 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:20 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:10 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 18:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 21:40 +0200
    Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:10 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Will Deacon <will.deacon@arm.com> - 2017-07-06 19:10 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:30 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:20 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:40 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 10:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 12:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 13:30 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 16:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-07 19:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 14:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 16:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-08 18:30 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-10 19:30 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:10 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 11:40 +0200
  [PATCH v3 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:30 +0200
    [PATCH v3 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
    [PATCH v3 5/9] exit: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
    [PATCH v3 8/9] locking: Remove spin_unlock_wait() generic definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200

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