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Date: Fri, 7 Jul 2017 11:32:49 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Laight <David.Laight@ACULAB.COM>,
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Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()
Message-ID: <20170707093249.3pskpxhf3ozfhrsr@gmail.com>
References: <20170629235918.GA6445@linux.vnet.ibm.com>
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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