On Sun, Oct 02, 2022 at 11:47:10PM +0200, Frederic Weisbecker wrote: > On Sun, Oct 02, 2022 at 09:09:57AM -0700, Paul E. McKenney wrote: > > On Sun, Oct 02, 2022 at 05:55:16PM +0200, Frederic Weisbecker wrote: > > > On Thu, Sep 29, 2022 at 11:07:25AM -0700, Paul E. McKenney wrote: > > > > @@ -1090,7 +1121,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, > > > > int ss_state; > > > > > > > > check_init_srcu_struct(ssp); > > > > - idx = srcu_read_lock(ssp); > > > > + idx = __srcu_read_lock_nmisafe(ssp); > > > > > > Why do we need to force the atomic based version here (even if CONFIG_NEED_SRCU_NMI_SAFE=y)? > > > > In kernels built with CONFIG_NEED_SRCU_NMI_SAFE=n, we of course need it. > > As you say, in kernels built with CONFIG_NEED_SRCU_NMI_SAFE=y, we don't. > > But it doesn't hurt to always use __srcu_read_lock_nmisafe() here, and > > this is nowhere near a fastpath, so there is little benefit to using > > __srcu_read_lock() when it is safe to do so. > > > > In addition, note that it is possible that a given srcu_struct structure's > > first grace period is executed before its first reader. In that > > case, we have no way of knowing which of __srcu_read_lock_nmisafe() > > or __srcu_read_lock() to choose. > > > > So this code always does it the slow(ish) safe way. > > But then srcu_read_lock_nmisafe() would work as well, right? Almost. The problem is that without the leading "__", this would convince SRCU that this is an NMI-safe srcu_struct. Which it might not be. Worse yet, if this srcu_struct had already done an srcu_read_lock(), it would splat. > > > > ss_state = smp_load_acquire(&ssp->srcu_size_state); > > > > if (ss_state < SRCU_SIZE_WAIT_CALL) > > > > sdp = per_cpu_ptr(ssp->sda, 0); > > > > @@ -1123,7 +1154,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, > > > > srcu_funnel_gp_start(ssp, sdp, s, do_norm); > > > > else if (needexp) > > > > srcu_funnel_exp_start(ssp, sdp_mynode, s); > > > > - srcu_read_unlock(ssp, idx); > > > > + __srcu_read_unlock_nmisafe(ssp, idx); > > > > return s; > > > > } > > > > > > > > @@ -1427,13 +1458,13 @@ void srcu_barrier(struct srcu_struct *ssp) > > > > /* Initial count prevents reaching zero until all CBs are posted. */ > > > > atomic_set(&ssp->srcu_barrier_cpu_cnt, 1); > > > > > > > > - idx = srcu_read_lock(ssp); > > > > + idx = __srcu_read_lock_nmisafe(ssp); > > > > > > And same here? > > > > Yes, same here. ;-) > > Now bonus question: why do SRCU grace period starting/tracking > need to be in an SRCU read side critical section? :o) Because I am lazy and like to keep things simple? ;-) More seriously, take a look at srcu_gp_start_if_needed() and the functions it calls and ask yourself what bad things could happen if they were preempted for an arbitrarily long period of time. Thanx, Paul
