From 96017bf9039763a2e02dcc6adaa18592cd73a39d Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 28 Jul 2021 10:53:41 -0700 Subject: [PATCH] rcu-tasks: Simplify trc_read_check_handler() atomic operations Currently, trc_wait_for_one_reader() atomically increments the trc_n_readers_need_end counter before sending the IPI invoking trc_read_check_handler(). All failure paths out of trc_read_check_handler() and also from the smp_call_function_single() within trc_wait_for_one_reader() must carefully atomically decrement this counter. This is more complex than it needs to be. This commit therefore simplifies things and saves a few lines of code by dispensing with the atomic decrements in favor of having trc_read_check_handler() do the atomic increment only in the success case. In theory, this represents no change in functionality. Signed-off-by: Paul E. McKenney --- kernel/rcu/tasks.h | 20 +++----------------- 1 file changed, 3 insertions(+), 17 deletions(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 3b2f8038064a..c9d8583ffe59 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -890,32 +890,24 @@ static void trc_read_check_handler(void *t_in) // If the task is no longer running on this CPU, leave. if (unlikely(texp != t)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); goto reset_ipi; // Already on holdout list, so will check later. } // If the task is not in a read-side critical section, and // if this is the last reader, awaken the grace-period kthread. if (likely(!READ_ONCE(t->trc_reader_nesting))) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); - // Mark as checked after decrement to avoid false - // positives on the above WARN_ON_ONCE(). WRITE_ONCE(t->trc_reader_checked, true); goto reset_ipi; } // If we are racing with an rcu_read_unlock_trace(), try again later. - if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); + if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) goto reset_ipi; - } WRITE_ONCE(t->trc_reader_checked, true); // Get here if the task is in a read-side critical section. Set // its state so that it will awaken the grace-period kthread upon // exit from that critical section. + atomic_inc(&trc_n_readers_need_end); // One more to wait on. WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)); WRITE_ONCE(t->trc_reader_special.b.need_qs, true); @@ -1015,21 +1007,15 @@ static void trc_wait_for_one_reader(struct task_struct *t, if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0) return; - atomic_inc(&trc_n_readers_need_end); per_cpu(trc_ipi_to_cpu, cpu) = true; t->trc_ipi_to_cpu = cpu; rcu_tasks_trace.n_ipis++; - if (smp_call_function_single(cpu, - trc_read_check_handler, t, 0)) { + if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) { // Just in case there is some other reason for // failure than the target CPU being offline. rcu_tasks_trace.n_ipis_fails++; per_cpu(trc_ipi_to_cpu, cpu) = false; t->trc_ipi_to_cpu = cpu; - if (atomic_dec_and_test(&trc_n_readers_need_end)) { - WARN_ON_ONCE(1); - wake_up(&trc_wait); - } } } }