diff options
| author | Paul E. McKenney | 2023-08-01 12:11:18 -0700 |
|---|---|---|
| committer | Paul E. McKenney | 2023-08-14 14:58:25 -0700 |
| commit | 9d0cce2bc3874dd03f7471ec00ae4acb5a77e43c (patch) | |
| tree | f814bbaa500d651ae6a23e775f665bd80d5cb639 /kernel | |
| parent | cb88f7f51bc6f351a529ff61d0a706c6eae1417a (diff) | |
rcu-tasks: Fix boot-time RCU tasks debug-only deadlock
In kernels built with CONFIG_PROVE_RCU=y (for example, lockdep kernels),
the following sequence of events can occur:
o rcu_init_tasks_generic() is invoked just before init is spawned.
It invokes rcu_spawn_tasks_kthread() and friends.
o rcu_spawn_tasks_kthread() invokes rcu_spawn_tasks_kthread_generic(),
which uses kthread_run() to create the needed kthread.
o Control returns to rcu_init_tasks_generic(), which, because this
is a CONFIG_PROVE_RCU=y kernel, invokes the version of the
rcu_tasks_initiate_self_tests() function that actually does
something, including invoking synchronize_rcu_tasks(), which
in turn invokes synchronize_rcu_tasks_generic().
o synchronize_rcu_tasks_generic() sees that the ->kthread_ptr is
still NULL, because the newly spawned kthread has not yet
started.
o The new kthread starts, preempting synchronize_rcu_tasks_generic()
just after its check. This kthread invokes rcu_tasks_one_gp(),
which acquires ->tasks_gp_mutex, and, seeing no work, blocks
in rcuwait_wait_event(). Note that this step requires either
a preemptible kernel or a fault-injection-style sleep at the
beginning of mutex_lock().
o synchronize_rcu_tasks_generic() resumes and invokes rcu_tasks_one_gp().
o rcu_tasks_one_gp() attempts to acquire ->tasks_gp_mutex, which
is still held by the newly spawned kthread's rcu_tasks_one_gp()
function. Deadlock.
Because the only reason for ->tasks_gp_mutex is to handle pre-kthread
synchronous grace periods, this commit avoids this deadlock by having
rcu_tasks_one_gp() momentarily release ->tasks_gp_mutex while invoking
rcuwait_wait_event(). This allows the call to rcu_tasks_one_gp() from
synchronize_rcu_tasks_generic() proceed.
Note that it is not necessary to release the mutex anywhere else in
rcu_tasks_one_gp() because rcuwait_wait_event() is the only function
that can block indefinitely.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Roy Hopkins <rhopkins@suse.de>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Roy Hopkins <rhopkins@suse.de>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/rcu/tasks.h | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index a4cd4ea2402c..5572f4a22996 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -570,10 +570,12 @@ static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot) if (unlikely(midboot)) { needgpcb = 0x2; } else { + mutex_unlock(&rtp->tasks_gp_mutex); set_tasks_gp_state(rtp, RTGS_WAIT_CBS); rcuwait_wait_event(&rtp->cbs_wait, (needgpcb = rcu_tasks_need_gpcb(rtp)), TASK_IDLE); + mutex_lock(&rtp->tasks_gp_mutex); } if (needgpcb & 0x2) { |