On Mon, Sep 21, 2015 at 11:37:47AM -0400, Alan Stern wrote: > On Mon, 21 Sep 2015, Felipe Balbi wrote: > > > On Mon, Sep 21, 2015 at 10:50:10AM -0400, Alan Stern wrote: > > > On Mon, 21 Sep 2015, Felipe Balbi wrote: > > > > > > > On Mon, Sep 21, 2015 at 10:31:15AM -0400, Alan Stern wrote: > > > > > On Mon, 21 Sep 2015, Roger Quadros wrote: > > > > > > > > > > > Using spin_lock() in hard irq handler is pointless > > > > > > and causes a BUG() in RT (real-time) configuration > > > > > > so get rid of it. > > > > > > > > > > Wait a minute. Who says spin_lock is pointless in an IRQ handler? > > > > > > > > in the top half IRQs are already disabled, how can this race ? > > > > > > It can race with code running on a different CPU. > > > > fair point. > > In fact, isn't this the main purpose of spinlocks? There's no point > using a spinlock to protect against races occurring on a single CPU, > because (in non-RT situations) the kernel can't schedule or preempt > while a spinlock is held, so no race can occur. The whole idea of > spinlocks is to protect against cross-CPU races. > > Now, maybe the spinlock usage that Roger is removing really is > unnecessary in this top-half handler. I don't know; I haven't looked > at the code. But in general, spinlocks are highly necessary in IRQ > handlers. this does raise some questions of what to do then ? We can't use spin_locks when in RT because they're reimplemented as mutexes and can sleep. Converting every single driver to raw_spin_locks defeats the purpose of having everything and their dogs preemptable. Difficult to say. Let's defer this patch until we can come up with a good answer to this. -- balbi
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