Re: MSI interrupts

On 3/24/2025 10:46 AM, EricP wrote:

Dan Cross wrote:

In article <fe9715fa347144df1e584463375107cf@www.novabbs.org>,
MitchAlsup1 <mitchalsup@aol.com> wrote:

On Thu, 20 Mar 2025 12:44:08 +0000, Dan Cross wrote:

>

Sometimes you really don't want to be interrupted.

And sometimes you don't want to be interrupted unless the
"house is on fire"; I cannot see a time when "the house is
on fire" that you would not want to take the interrupt.
>
Is there one ?!?

>
Consider a thread that takes a spinlock; suppose some
high-priority interrupt comes in while the thread is holding
that lock.  In response to the interrupt, software decides to
suspend the thread and switch some other thread; that thread
wants to lock the spin lock that the now-descheduled thread is
holding: a classic deadlock scenario.

 Terminology: mutexes coordinate mutual exclusion between threads,
spinlocks coordinate mutual exclusion between cpu cores.
Windows "critical sections" are mutexes with a fast path.

[...]
Fwiw, they also have a "spin count", which might spin a couple of times on contention before they wait in the kernel. Trying to spin to gain a fast path before waiting in the kernel, wrt userspace logic. PAUSE instruction comes to mind. Iirc, they are called adaptive mutexes.