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

From: Ingo Molnar
Date: Fri Jul 07 2017 - 05:33:05 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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);

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! ;-)