On 30.07.21 09:14, Bezdeka, Florian (T RDA IOT SES-DE) wrote: > On Thu, 2021-07-29 at 19:29 +0200, Jan Kiszka wrote: >> On 29.07.21 18:22, Bezdeka, Florian (T RDA IOT SES-DE) wrote: >>> On Thu, 2021-07-29 at 17:45 +0200, Jan Kiszka wrote: >>>> On 29.07.21 17:06, Philippe Gerum wrote: >>>>> >>>>> Florian Bezdeka <florian.bezd...@siemens.com> writes: >>>>> >>>>>> According to the POSIX spec the value of the timeout parameter needs >>>>>> not to be validated if the mutex/semaphore could be taken immediately. >>>>>> >>>>>> While the implementation of the semaphore timedwait (sem_timedwait()) >>>>>> allowed an invalid timeout pthread_mutex_timedlock() was failing with >>>>>> -EFAULT in case the mutex could be taken immediately. >>>>>> >>>>>> Signed-off-by: Florian Bezdeka <florian.bezd...@siemens.com> >>>>>> --- >>>>>> >>>>>> This was detected while preparing y2038 stuff. Not sure if that should >>>>>> go into 3.2. Comments welcome... >>>>>> >>>>> >>>>> upsides: save a few cycles in the non-contended case by not copying in >>>>> the timeout from userland uselessly + might help passing some picky >>>>> POSIX compliance tests. >>>>> >>>>> downside: adds an extra atomic op in the contended case. >>>>> >>>> >>>> How do we compare to libc implementations here? >>> >>> I changed the validation done on cobalt side, not libcobalt. So what >>> exactly do you mean? >>> >> >> Indeed, I was looking for the fast or almost fast paths in userspace. >> >> But now I'm confused: The uncontended case is generally not supposed to >> trigger a syscall. That is always the slow path anyway, e.g. because ned >> to migrated first (likely how you were able to trigger the issue?). > > The fast-path was always fine, that is right. But please note that > depending on the kernel configuration (especially with > CONFIG_XENO_OPT_DEBUG_MUTEX_SLEEP enabled) we will always be forced > into the slow-path. > > So we had another imbalance depending on the kernel configuration. >
OK, I just wanted to understand whether that single atomic more is an issue - it's not at all, we are slow already at that point. Jan -- Siemens AG, T RDA IOT Corporate Competence Center Embedded Linux
