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

Ankur Arora
2025-02-03T21:49:42Z

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!

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(+)

--
2.43.5