Ankur Arora <ankur.a.arora@xxxxxxxxxx> 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@xxxxxxxxxx/ 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@xxxxxxxxxx/ > [2] https://lore.kernel.org/lkml/20250107140004.2732830-9-memxor@xxxxxxxxx/ > > -- > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Will Deacon <will@xxxxxxxxxx> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> > Cc: linux-arch@xxxxxxxxxxxxxxx > > 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
