diff options
author | Paul E. McKenney <paulmck@kernel.org> | 2021-09-23 10:07:14 -0700 |
---|---|---|
committer | Paul E. McKenney <paulmck@kernel.org> | 2021-11-30 17:28:16 -0800 |
commit | 1f8da406a964dcc01121385a54fafd28e1c6a796 (patch) | |
tree | 9314a34910faa1d92fb2f877c94f94818e24e613 /kernel/rcu/Kconfig | |
parent | 300c0c5e721834f484b03fa3062602dd8ff48413 (diff) |
srcu: Prevent redundant __srcu_read_unlock() wakeup
Tiny SRCU readers can appear at task level, but also in interrupt and
softirq handlers. Because Tiny SRCU is selected only in kernels built
with CONFIG_SMP=n and CONFIG_PREEMPTION=n, it is not possible for a grace
period to start while there is a non-task-level SRCU reader executing.
This means that it does not make sense for __srcu_read_unlock() to awaken
the Tiny SRCU grace period, because that can only happen when the grace
period is waiting for one value of ->srcu_idx and __srcu_read_unlock()
is ending the last reader for some other value of ->srcu_idx. After all,
any such wakeup will be redundant.
Worse yet, in some cases, such wakeups generate lockdep splats:
======================================================
WARNING: possible circular locking dependency detected
5.15.0-rc1+ #3758 Not tainted
------------------------------------------------------
rcu_torture_rea/53 is trying to acquire lock:
ffffffff9514e6a8 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}, at:
xa/0x30
but task is already holding lock:
ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at:
_extend+0x370/0x400
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&p->pi_lock){-.-.}-{2:2}:
_raw_spin_lock_irqsave+0x2f/0x50
try_to_wake_up+0x50/0x580
swake_up_locked.part.7+0xe/0x30
swake_up_one+0x22/0x30
rcutorture_one_extend+0x1b6/0x400
rcu_torture_one_read+0x290/0x5d0
rcu_torture_timer+0x1a/0x70
call_timer_fn+0xa6/0x230
run_timer_softirq+0x493/0x4c0
__do_softirq+0xc0/0x371
irq_exit+0x73/0x90
sysvec_apic_timer_interrupt+0x63/0x80
asm_sysvec_apic_timer_interrupt+0x12/0x20
default_idle+0xb/0x10
default_idle_call+0x5e/0x170
do_idle+0x18a/0x1f0
cpu_startup_entry+0xa/0x10
start_kernel+0x678/0x69f
secondary_startup_64_no_verify+0xc2/0xcb
-> #0 (srcu_ctl.srcu_wq.lock){..-.}-{2:2}:
__lock_acquire+0x130c/0x2440
lock_acquire+0xc2/0x270
_raw_spin_lock_irqsave+0x2f/0x50
swake_up_one+0xa/0x30
rcutorture_one_extend+0x387/0x400
rcu_torture_one_read+0x290/0x5d0
rcu_torture_reader+0xac/0x200
kthread+0x12d/0x150
ret_from_fork+0x22/0x30
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&p->pi_lock);
lock(srcu_ctl.srcu_wq.lock);
lock(&p->pi_lock);
lock(srcu_ctl.srcu_wq.lock);
*** DEADLOCK ***
1 lock held by rcu_torture_rea/53:
#0: ffff95c642479d80 (&p->pi_lock){-.-.}-{2:2}, at:
_extend+0x370/0x400
stack backtrace:
CPU: 0 PID: 53 Comm: rcu_torture_rea Not tainted 5.15.0-rc1+
Hardware name: Red Hat KVM/RHEL-AV, BIOS
e_el8.5.0+746+bbd5d70c 04/01/2014
Call Trace:
check_noncircular+0xfe/0x110
? find_held_lock+0x2d/0x90
__lock_acquire+0x130c/0x2440
lock_acquire+0xc2/0x270
? swake_up_one+0xa/0x30
? find_held_lock+0x72/0x90
_raw_spin_lock_irqsave+0x2f/0x50
? swake_up_one+0xa/0x30
swake_up_one+0xa/0x30
rcutorture_one_extend+0x387/0x400
rcu_torture_one_read+0x290/0x5d0
rcu_torture_reader+0xac/0x200
? rcutorture_oom_notify+0xf0/0xf0
? __kthread_parkme+0x61/0x90
? rcu_torture_one_read+0x5d0/0x5d0
kthread+0x12d/0x150
? set_kthread_struct+0x40/0x40
ret_from_fork+0x22/0x30
This is a false positive because there is only one CPU, and both locks
are raw (non-preemptible) spinlocks. However, it is worthwhile getting
rid of the redundant wakeup, which has the side effect of breaking
the theoretical deadlock cycle. This commit therefore eliminates the
redundant wakeups.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/rcu/Kconfig')
0 files changed, 0 insertions, 0 deletions