Messages in this thread Patch in this message | | | Date | Sat, 17 Apr 2021 15:16:49 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/2] srcu: Fix broken node geometry after early ssp init |
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On Wed, Apr 14, 2021 at 08:55:38AM -0700, Paul E. McKenney wrote: > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > index 75ed367d5b60..24db97cbf76b 100644 > > --- a/kernel/rcu/rcu.h > > +++ b/kernel/rcu/rcu.h > > @@ -278,6 +278,7 @@ extern void resched_cpu(int cpu); > > extern int rcu_num_lvls; > > extern int num_rcu_lvl[]; > > extern int rcu_num_nodes; > > +extern bool rcu_geometry_initialized; > > Can this be a static local variable inside rcu_init_geometry()? > > After all, init_srcu_struct() isn't called all that often, and its overhead > is such that an extra function call and check is going to hurt it. This > of course requires removing __init from rcu_init_geometry(), but it is not > all that large, so why not just remove the __init? > > But if we really are worried about reclaiming rcu_init_geometry()'s > instructions (maybe we are?), then rcu_init_geometry() can be split > into a function that just does the check (which is not __init) and the > remainder of the function, which could remain __init.
There you go:
--- From: Frederic Weisbecker <frederic@kernel.org> Date: Wed, 31 Mar 2021 16:10:36 +0200 Subject: [PATCH] srcu: Fix broken node geometry after early ssp init
An ssp initialized before rcu_init_geometry() will have its snp hierarchy based on CONFIG_NR_CPUS.
Once rcu_init_geometry() is called, the nodes distribution is shrinked and optimized toward meeting the actual possible number of CPUs detected on boot.
Later on, the ssp that was initialized before rcu_init_geometry() is confused and sometimes refers to its initial CONFIG_NR_CPUS based node hierarchy, sometimes to the new num_possible_cpus() based one instead. For example each of its sdp->mynode remain backward and refer to the early node leaves that may not exist anymore. On the other hand the srcu_for_each_node_breadth_first() refers to the new node hierarchy.
There are at least two bad possible outcomes to this:
1) a) A callback enqueued early on an 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() which shrinks the nodes level to a single one. srcu_gp_end() walks through the new snp 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 ssp that gets 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.
Solve this with initializing nodes geometry whenever an struct srcu_state happens to be initialized before rcu_init(). This way we make sure the RCU nodes hierarchy is properly built and distributed before the nodes of an struct srcu_state are allocated.
Suggested-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Neeraj Upadhyay <neeraju@codeaurora.org> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: Uladzislau Rezki <urezki@gmail.com> --- kernel/rcu/rcu.h | 2 ++ kernel/rcu/srcutree.c | 3 +++ kernel/rcu/tree.c | 17 ++++++++++++++++- 3 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 75ed367d5b60..edba5976ca17 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 55bc9473562b..9efd5eeb5dd5 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; + /* First make sure the nodes hierarchy is properly built */ + 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 51600ef3fb0c..906f81786ca5 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4617,11 +4617,26 @@ 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) { + /* + * Arrange for warning if rcu_init_geometry() was called before + * setup_nr_cpu_ids(). We may miss cases when + * nr_cpus_ids == NR_CPUS but that shouldn't matter too much. + */ + 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. -- 2.25.1
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