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Urgent RCU pull request for v5.17

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This pull request fixes a math error added in 7a30871b6a ("rcu-tasks:
 Introduce ->percpu_enqueue_shift for dynamic queue selection') during the
 v5.17 merge window.  This commit works correctly only on systems with a
 power-of-two number of CPUs, which just so happens to be the kind that
 rcutorture always uses by default.
 
 This pull request fixes the math so that things also work on systems
 that don't happen to have a power-of-two number of CPUs.
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Merge tag 'rcu-urgent.2022.01.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU fix from Paul McKenney:
 "This fixes a brown-paper-bag bug in RCU tasks that causes things like
  BPF and ftrace to fail miserably on systems with non-power-of-two
  numbers of CPUs.

  It fixes a math error added in 7a30871b6a ("rcu-tasks: Introduce
  ->percpu_enqueue_shift for dynamic queue selection') during the v5.17
  merge window. This commit works correctly only on systems with a
  power-of-two number of CPUs, which just so happens to be the kind that
  rcutorture always uses by default.

  This pull request fixes the math so that things also work on systems
  that don't happen to have a power-of-two number of CPUs"

* tag 'rcu-urgent.2022.01.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  rcu-tasks: Fix computation of CPU-to-list shift counts
This commit is contained in:
Linus Torvalds 2022-01-28 19:19:22 +02:00
commit a773abf72e
1 changed files with 8 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
kernel/rcu/tasks.h
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@ -123,7 +123,7 @@ static struct rcu_tasks rt_name = \
.call_func = call, \
.rtpcpu = &rt_name ## __percpu, \
.name = n, \
.percpu_enqueue_shift = ilog2(CONFIG_NR_CPUS), \
.percpu_enqueue_shift = ilog2(CONFIG_NR_CPUS) + 1, \
.percpu_enqueue_lim = 1, \
.percpu_dequeue_lim = 1, \
.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex), \
@ -216,6 +216,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
int cpu;
unsigned long flags;
int lim;
int shift;
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rcu_task_enqueue_lim < 0) {
@ -229,7 +230,10 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
if (lim > nr_cpu_ids)
lim = nr_cpu_ids;
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids / lim));
shift = ilog2(nr_cpu_ids / lim);
if (((nr_cpu_ids - 1) >> shift) >= lim)
shift++;
WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
smp_store_release(&rtp->percpu_enqueue_lim, lim);
for_each_possible_cpu(cpu) {
@ -298,7 +302,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
if (unlikely(needadjust)) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids));
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids) + 1);
WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
@ -413,7 +417,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim > 1) {
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids));
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids) + 1);
smp_store_release(&rtp->percpu_enqueue_lim, 1);
rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);