Hi Kai, On Wed, Nov 9, 2011 at 1:07 PM, Kai Meyer <kai@gnukai.com> wrote:
When I readup on spinlocks, it seems like I need to choose between disabling interrupts and not. If a spinlock_t is never used during an interrupt, am I safe to leave interrupts enabled while I hold the lock? (Same question for read/write locks if it is different.)
So the intention behind using a spinlock is to provide mutual exclusion. A spinlock by itself only really provides mutual exclusion between 2 cores, and not within the same core. To provide the mutual exclusion within the same core, you need to disable interrupts. Normally, you would disable interrupts and acquire the spinlock to guarantee that mutual exclusion, and the only reason you would normally use the spinlock without disabling interrupts is when you know that interrupts are already disabled. The danger of acquiring a spinlock with interrupts enabled is that if another interrupt fired (or the same interrupt fired again) and it tried to acquire the same spinlock, then you could have deadlock. If no interrupts touch the spinlock, then you're probably using the wrong mutual exclusion mechanism. spinlocks are really intended to provide mutual exclsion between interrupt context and non-interrupt context. Also remember, that on a non-SMP (aka UP) build, spinlocks become no-ops (except when certain debug checking code is enabled). -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com