Messages in this thread Patch in this message | | | Date | Fri, 7 Jul 2017 11:32:49 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 0/9] Remove spin_unlock_wait() |
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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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