On Fri, Mar 28, 2008 at 11:01:24AM +1100, Nick Piggin wrote:Ah, I see what you're saying. The deadlock would be (on a single CPU machine), task A holding the semaphore, being preempted, task B taking a spinlock (thus non-preemptable), then calling down_spin() which will never succeed. That hadn't occurred to me -- I'm not used to thinking about preemption. I considered interrupt context and saw how that would deadlock, so just put a note in the documentation that it wasn't usable from interrupts. So it makes little sense to add this to semaphores. Better to introduce a spinaphore, as you say. I think we can do better here with: atomic_set(max); and while (unlikely(!atomic_add_unless(&ss->cur, -1, 0))) while (atomic_read(&ss->cur) == 0) cpu_relax(); It still spins on the spinaphore itself rather than on a local cacheline, so there's room for improvement. But it's not clear whether it'd be worth it. LOC wasn't really my argument -- I didn't want to introduce a new data structure unnecessarily. But the pitfalls (that I hadn't seen) of mixing down_spin() into semaphores are just too awful. I'll pop this patch off the stack of semaphore patches. Thanks. -- Intel are signing my paycheques ... these opinions are still mine "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." --
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| Tarkan Erimer | Re: Dual-Licensing Linux Kernel with GPL V2 and GPL V3 |
| Greg Kroah-Hartman | [PATCH 001/196] Chinese: Add the known_regression URI to the HOWTO |
git: | |
| Gerrit Renker | [PATCH 27/37] dccp: Integration of dynamic feature activation - part 2 (server side) |
| Jarek Poplawski | Re: [PATCH] pkt_sched: Destroy gen estimators under rtnl_lock(). |
| David Miller | [GIT]: Networking |
| Matheos Worku | 2.6.24 BUG: soft lockup - CPU#X |
