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. > > 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. ;-) Thanx, Paul > Thanks. > > > if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER) > > srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, 0)); > > else > > for_each_possible_cpu(cpu) > > srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu)); > > - srcu_read_unlock(ssp, idx); > > + __srcu_read_unlock_nmisafe(ssp, idx); > > > > /* Remove the initial count, at which point reaching zero can happen. */ > > if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt)) > > -- > > 2.31.1.189.g2e36527f23 > >
