The current Tiny SRCU implementation of srcu_read_unlock() awakens the grace-period processing when exiting the outermost SRCU read-side critical section. However, not all Linux-kernel configurations and contexts permit swake_up_one() to be invoked while interrupts are disabled, and this can result in indefinitely extended SRCU grace periods. This commit therefore only invokes swake_up_one() when interrupts are enabled, and introduces polling to the grace-period workqueue handler.
Reported-by: kernel test robot <[email protected]> Reported-by: Zqiang <[email protected]> Closes: https://lore.kernel.org/oe-lkp/[email protected] Signed-off-by: Paul E. McKenney <[email protected]> --- kernel/rcu/srcutiny.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c index e3b64a5e0ec7..3450c3751ef7 100644 --- a/kernel/rcu/srcutiny.c +++ b/kernel/rcu/srcutiny.c @@ -106,15 +106,15 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx) newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1; WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval); preempt_enable(); - if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task()) + if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task() && !irqs_disabled()) swake_up_one(&ssp->srcu_wq); } EXPORT_SYMBOL_GPL(__srcu_read_unlock); /* * Workqueue handler to drive one grace period and invoke any callbacks - * that become ready as a result. Single-CPU and !PREEMPTION operation - * means that we get away with murder on synchronization. ;-) + * that become ready as a result. Single-CPU operation and preemption + * disabling mean that we get away with murder on synchronization. ;-) */ void srcu_drive_gp(struct work_struct *wp) { @@ -141,7 +141,12 @@ void srcu_drive_gp(struct work_struct *wp) WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); WRITE_ONCE(ssp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */ preempt_enable(); - swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx])); + do { + // Deadlock issues prevent __srcu_read_unlock() from + // doing an unconditional wakeup, so polling is required. + swait_event_timeout_exclusive(ssp->srcu_wq, + !READ_ONCE(ssp->srcu_lock_nesting[idx]), HZ / 10); + } while (READ_ONCE(ssp->srcu_lock_nesting[idx])); preempt_disable(); // Needed for PREEMPT_LAZY WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */ WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); -- 2.40.1
