From 635bdb9d22796fc6ba1958ec73ab0a0a693c58d6 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 8 May 2025 16:45:01 -0700 Subject: [PATCH] rcutorture: Start rcu_torture_writer() after rcu_torture_reader() Testing of rcutorture's SRCU-P scenario on a large arm64 system resulted in rcu_torture_writer() forward-progress failures, but these same tests passed on x86. After some off-list discussion of possible memory-ordering causes for these failures, Boqun showed that these were in fact due to reordering, but by the scheduler, not by the memory system. On x86, rcu_torture_writer() would have run quickly enough that by the time the rcu_torture_updown() kthread started, the rcu_torture_current variable would already be initialized, thus avoiding a bug in which a NULL value would cause rcu_torture_updown() to do an extra call to srcu_up_read_fast(). This commit therefore moves creation of the rcu_torture_writer() kthread after that of the rcu_torture_reader() kthreads. This results in deterministic failures on x86. What about the double-srcu_up_read_fast() bug? Boqun has the fix. But let's also fix the test while we are at it! Reported-by: Joel Fernandes Reported-by: Boqun Feng Signed-off-by: Paul E. McKenney Signed-off-by: Joel Fernandes Signed-off-by: Neeraj Upadhyay (AMD) --- kernel/rcu/rcutorture.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c index d1e0d61d88152..a209d2419cfdb 100644 --- a/kernel/rcu/rcutorture.c +++ b/kernel/rcu/rcutorture.c @@ -4246,11 +4246,6 @@ rcu_torture_init(void) /* Start up the kthreads. */ rcu_torture_write_types(); - firsterr = torture_create_kthread(rcu_torture_writer, NULL, - writer_task); - if (torture_init_error(firsterr)) - goto unwind; - if (nrealfakewriters > 0) { fakewriter_tasks = kcalloc(nrealfakewriters, sizeof(fakewriter_tasks[0]), @@ -4283,6 +4278,12 @@ rcu_torture_init(void) if (torture_init_error(firsterr)) goto unwind; } + + firsterr = torture_create_kthread(rcu_torture_writer, NULL, + writer_task); + if (torture_init_error(firsterr)) + goto unwind; + nrealnocbers = nocbs_nthreads; if (WARN_ON(nrealnocbers < 0)) nrealnocbers = 1; -- 2.47.2