From b5befe842e6612cf894cf4a199924ee872d8b7d8 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Sat, 17 Apr 2021 15:16:49 +0200 Subject: [PATCH] srcu: Fix broken node geometry after early ssp init An srcu_struct structure that is initialized before rcu_init_geometry() will have its srcu_node hierarchy based on CONFIG_NR_CPUS. Once rcu_init_geometry() is called, this hierarchy is compressed as needed for the actual maximum number of CPUs for this system. Later on, that srcu_struct structure is confused, sometimes referring to its initial CONFIG_NR_CPUS-based hierarchy, and sometimes instead to the new num_possible_cpus() hierarchy. For example, each of its ->mynode fields continues to reference the original leaf rcu_node structures, some of which might no longer exist. On the other hand, srcu_for_each_node_breadth_first() traverses to the new node hierarchy. There are at least two bad possible outcomes to this: 1) a) A callback enqueued early on an srcu_data structure (call it *sdp) is recorded pending on sdp->mynode->srcu_data_have_cbs in srcu_funnel_gp_start() with sdp->mynode pointing to a deep leaf (say 3 levels). b) The grace period ends after rcu_init_geometry() shrinks the nodes level to a single one. srcu_gp_end() walks through the new srcu_node hierarchy without ever reaching the old leaves so the callback is never executed. This is easily reproduced on an 8 CPUs machine with CONFIG_NR_CPUS >= 32 and "rcupdate.rcu_self_test=1". The srcu_barrier() after early tests verification never completes and the boot hangs: [ 5413.141029] INFO: task swapper/0:1 blocked for more than 4915 seconds. [ 5413.147564] Not tainted 5.12.0-rc4+ #28 [ 5413.151927] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 5413.159753] task:swapper/0 state:D stack: 0 pid: 1 ppid: 0 flags:0x00004000 [ 5413.168099] Call Trace: [ 5413.170555] __schedule+0x36c/0x930 [ 5413.174057] ? wait_for_completion+0x88/0x110 [ 5413.178423] schedule+0x46/0xf0 [ 5413.181575] schedule_timeout+0x284/0x380 [ 5413.185591] ? wait_for_completion+0x88/0x110 [ 5413.189957] ? mark_held_locks+0x61/0x80 [ 5413.193882] ? mark_held_locks+0x61/0x80 [ 5413.197809] ? _raw_spin_unlock_irq+0x24/0x50 [ 5413.202173] ? wait_for_completion+0x88/0x110 [ 5413.206535] wait_for_completion+0xb4/0x110 [ 5413.210724] ? srcu_torture_stats_print+0x110/0x110 [ 5413.215610] srcu_barrier+0x187/0x200 [ 5413.219277] ? rcu_tasks_verify_self_tests+0x50/0x50 [ 5413.224244] ? rdinit_setup+0x2b/0x2b [ 5413.227907] rcu_verify_early_boot_tests+0x2d/0x40 [ 5413.232700] do_one_initcall+0x63/0x310 [ 5413.236541] ? rdinit_setup+0x2b/0x2b [ 5413.240207] ? rcu_read_lock_sched_held+0x52/0x80 [ 5413.244912] kernel_init_freeable+0x253/0x28f [ 5413.249273] ? rest_init+0x250/0x250 [ 5413.252846] kernel_init+0xa/0x110 [ 5413.256257] ret_from_fork+0x22/0x30 2) An srcu_struct structure that is initialized before rcu_init_geometry() and used afterward will always have stale rdp->mynode references, resulting in callbacks to be missed in srcu_gp_end(), just like in the previous scenario. This commit therefore causes init_srcu_struct_nodes to initialize the geometry, if needed. This ensures that the srcu_node hierarchy is properly built and distributed from the get-go. Suggested-by: Paul E. McKenney Signed-off-by: Frederic Weisbecker Cc: Boqun Feng Cc: Lai Jiangshan Cc: Neeraj Upadhyay Cc: Josh Triplett Cc: Joel Fernandes Cc: Uladzislau Rezki Signed-off-by: Paul E. McKenney --- kernel/rcu/rcu.h | 2 ++ kernel/rcu/srcutree.c | 3 +++ kernel/rcu/tree.c | 16 +++++++++++++++- 3 files changed, 20 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index ca3f2af32bf8..28de768bd875 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -308,6 +308,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt) } } +extern void rcu_init_geometry(void); + /* Returns a pointer to the first leaf rcu_node structure. */ #define rcu_first_leaf_node() (rcu_state.level[rcu_num_lvls - 1]) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 9ec35c158740..5fdbde687feb 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -90,6 +90,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp) struct srcu_node *snp; struct srcu_node *snp_first; + /* Initialize geometry if it has not already been initialized. */ + rcu_init_geometry(); + /* Work out the overall tree geometry. */ ssp->level[0] = &ssp->node[0]; for (i = 1; i < rcu_num_lvls; i++) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index c35b15229cff..0420b23fc9d0 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4582,11 +4582,25 @@ static void __init rcu_init_one(void) * replace the definitions in tree.h because those are needed to size * the ->node array in the rcu_state structure. */ -static void __init rcu_init_geometry(void) +void rcu_init_geometry(void) { ulong d; int i; + static unsigned long old_nr_cpu_ids; int rcu_capacity[RCU_NUM_LVLS]; + static bool initialized; + + if (initialized) { + /* + * Warn if setup_nr_cpu_ids() had not yet been invoked, + * unless nr_cpus_ids == NR_CPUS, in which case who cares? + */ + WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids); + return; + } + + old_nr_cpu_ids = nr_cpu_ids; + initialized = true; /* * Initialize any unspecified boot parameters.