Spinlocks and interrupts

[rohan puri]

On Thu, Nov 10, 2011 at 3:10 AM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Kai,
>
> On Wed, Nov 9, 2011 at 1:07 PM, Kai Meyer <kai at 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
>
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Nice explanation Dave.

Regards,
Rohan Puri
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