Re: [PATCH 0/4] barrier: Introduce smp_cond_load_*_timeout()

Kumar Kartikeya Dwivedi
2025-02-06T10:57:59Z

On Mon, 3 Feb 2025 at 22:49, Ankur Arora <ankur.a.arora@oracle.com> wrote:
>
> Hi,
>
> This series adds waited variants of the smp_cond_load() primitives:
> smp_cond_load_relaxed_timewait(), and smp_cond_load_acquire_timewait().
>
> There are two known users for these interfaces:
>
> - poll_idle() [1]
> - resilient queued spinlocks [2]
>
> For both of these cases we want to wait on a condition but also want
> to terminate the wait at some point.
>
> Now, in theory, that can be worked around by making the time check a
> part of the conditional expression provided to smp_cond_load_*():
>
> smp_cond_load_relaxed(&cvar, !VAL || time_check());
>
> That approach, however, runs into two problems:
>
> - smp_cond_load_*() only allow waiting on a condition: this might
> be okay when we are synchronously spin-waiting on the condition,
> but not on architectures where are actually waiting for a store
> to a cacheline.
>
> - this semantic problem becomes a real problem on arm64 if the
> event-stream is disabled. That means that there will be no
> asynchronous event (the event-stream) that periodically wakes
> the waiter, which might lead to an interminable wait if VAL is
> never written to.
>
> This series extends the smp_cond_load_*() interfaces by adding two
> arguments: a time-check expression and its associated time limit.
> This is sufficient to allow for both a synchronously waited
> implementation (like the generic cpu_relax() based loop), or one
> where the CPU waits for a store to a cacheline with an out-of-band
> timer.
>
> Any comments appreciated!
>

Thanks for splitting this and sending it out.
+cc bpf@ for visibility (please keep it in cc for subsequent versions).

>
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