Re: MSI interrupts

On 3/24/2025 10:26 PM, Stefan Monnier wrote:

It means that if a thread owning a lock is interrupted, that
thread gives the lock back--so there is no priority inversion
and no lock-starvation. Associated with the give back, and the
IP of the interrupted thread no longer points inside the
critical section.

 I think the discussion is a bit confusing because we have 2 different
notions of "critical section" at play, here:

Yup. User space mutex impls vs kernel space ones (sync algos, mutexes, ect...), that can be different in the way they are implemented. Using an XCHG instruction can give you a spinlock, mutex, ect... CMPXCHG and XCHG are very fun, never forget about XADD. In user space we only want to resort to the kernel to wait when we have to... We try to minimize that as the slow path, the last resort. The fast path can get around calling into the kernel. Spinning in user space on a contended mutex is okay as long as its rather limited. Then it can be called an adaptive mutex.
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