Messages in this thread | | | Date | Mon, 30 Nov 2009 08:39:23 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [rfc] "fair" rw spinlocks |
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On Mon, Nov 30, 2009 at 08:07:16AM -0800, Linus Torvalds wrote: > > > On Mon, 30 Nov 2009, Nick Piggin wrote: > > > > Well the simple thing I tried earlier was a per-cpu array of nesting > > counter there. It's not _too_ expensive, but it does add another cacheline > > access and branch there. It seems to work in solving the livelock though. > > So how did you do the nesting counter? Afaik, it needs to be something > like > > local_irq_save(flags); > if (!get_cpu_var(tasklist_counter)++) > spin_lock(&tasklist_lock); > local_irq_restore(flags); > > on the read_lock side (and the same in reverse on unlock). Which seems > quite a bit more expensive than what we have now. Especially on UP, but I > guess you can make it conditional on CONFIG_SMP (but that won't help > generic kernels).
My suggestion would be to put the nesting counter in the task structure to avoid this problem.
Thanx, Paul
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