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

[Ankur Arora <ankur.a.arora@oracle.com>]

Ankur Arora <ankur.a.arora@oracle.com> writes:

> 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]

The next version of the poll_idle() series (goes on top of this one):
 https://lore.kernel.org/lkml/20250218213337.377987-1-ankur.a.arora@oracle.com/

Ankur

>  - 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!
>
>
> Ankur
>
> [1] https://lore.kernel.org/lkml/20241107190818.522639-3-ankur.a.arora@oracle.com/
> [2] https://lore.kernel.org/lkml/20250107140004.2732830-9-memxor@gmail.com/
>
> --
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Cc: linux-arch@vger.kernel.org
>
> Ankur Arora (4):
>   asm-generic: barrier: Add smp_cond_load_relaxed_timewait()
>   asm-generic: barrier: Add smp_cond_load_acquire_timewait()
>   arm64: barrier: Add smp_cond_load_relaxed_timewait()
>   arm64: barrier: Add smp_cond_load_acquire_timewait()
>
>  arch/arm64/include/asm/barrier.h | 74 ++++++++++++++++++++++++++++++++
>  include/asm-generic/barrier.h    | 71 ++++++++++++++++++++++++++++++
>  2 files changed, 145 insertions(+)


--
ankur