Commit 58ac42f2 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Frederic Weisbecker
Browse files

srcu: Permit Tiny SRCU srcu_read_unlock() with interrupts disabled 



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: default avatarkernel test robot <oliver.sang@intel.com>
Reported-by: default avatarZqiang <qiang.zhang@linux.dev>
Closes: https://lore.kernel.org/oe-lkp/202508261642.b15eefbb-lkp@intel.com


Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
parent 3a866087

kernel/rcu/srcutiny.c

+9 −4
Original line number Diff line number Diff line
@@ -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);