Quoting Douglas Anderson (2020-04-13 10:04:14) > Auditing tcs_invalidate() made me worried. Specifically I saw that it > used spin_lock(), not spin_lock_irqsave(). That always worries me > unless I can trace for sure that I'm in the interrupt handler or that > someone else already disabled interrupts. > > Looking more at it, there is actually no reason for these locks > anyway. Specifically the only reason you'd ever call > rpmh_rsc_invalidate() is if you cared that the sleep/wake TCSes were > empty. That means that they need to continue to be empty even after > rpmh_rsc_invalidate() returns. The only way that can happen is if the > caller already has done something to keep all other RPMH users out. > It should be noted that even though the caller is only worried about > making sleep/wake TCSes empty, they also need to worry about stopping > active-only transfers if they need to handle the case where > active-only transfers might borrow the wake TCS. > > At the moment rpmh_rsc_invalidate() is only called in PM code from the > last CPU. If that later changes the caller will still need to solve > the above problems themselves, so these locks will never be useful. > > Continuing to audit tcs_invalidate(), I found a bug. The function > didn't properly check for a borrowed TCS if we hadn't recently written > anything into the TCS. Specifically, if we've never written to the > WAKE_TCS (or we've flushed it recently) then tcs->slots is empty. > We'll early-out and we'll never call tcs_is_free(). > > I thought about fixing this bug by either deleting the early check for > bitmap_empty() or possibly only doing it if we knew we weren't on a > TCS that could be borrowed. However, I think it's better to just > delete the checks. > > As argued above it's up to the caller to make sure that all other > users of RPMH are quiet before tcs_invalidate() is called. Since > callers need to handle the zero-active-TCS case anyway that means they > need to make sure that the active-only transfers are quiet before > calling too. The one way tcs_invalidate() gets called today is > through rpmh_rsc_cpu_pm_callback() which calls > rpmh_rsc_ctrlr_is_busy() to handle this. When we have another path to > get to tcs_invalidate() it will also need to come up with something > similar and it won't need this extra check either. If we later find > some code path that actually needs this check back in (and somehow > manages to be race free) we can always add it back in. > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > Reviewed-by: Maulik Shah <mkshah@xxxxxxxxxxxxxx> > Tested-by: Maulik Shah <mkshah@xxxxxxxxxxxxxx> > --- Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
