6.12-stable review patch. If anyone has any objections, please let me know.
------------------
From: Paul E. McKenney paulmck@kernel.org
[ Upstream commit 90c09d57caeca94e6f3f87c49e96a91edd40cbfd ]
On kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() is invoked within an interrupts-disabled region of code [1], it will invoke rcu_read_unlock_special(), which uses an irq-work handler to force the system to notice when the RCU read-side critical section actually ends. That end won't happen until interrupts are enabled at the soonest.
In some kernels, such as those booted with rcutree.use_softirq=y, the irq-work handler is used unconditionally.
The per-CPU rcu_data structure's ->defer_qs_iw_pending field is updated by the irq-work handler and is both read and updated by rcu_read_unlock_special(). This resulted in the following KCSAN splat:
------------------------------------------------------------------------
BUG: KCSAN: data-race in rcu_preempt_deferred_qs_handler / rcu_read_unlock_special
read to 0xffff96b95f42d8d8 of 1 bytes by task 90 on cpu 8: rcu_read_unlock_special+0x175/0x260 __rcu_read_unlock+0x92/0xa0 rt_spin_unlock+0x9b/0xc0 __local_bh_enable+0x10d/0x170 __local_bh_enable_ip+0xfb/0x150 rcu_do_batch+0x595/0xc40 rcu_cpu_kthread+0x4e9/0x830 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30
write to 0xffff96b95f42d8d8 of 1 bytes by task 88 on cpu 8: rcu_preempt_deferred_qs_handler+0x1e/0x30 irq_work_single+0xaf/0x160 run_irq_workd+0x91/0xc0 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30
no locks held by irq_work/8/88. irq event stamp: 200272 hardirqs last enabled at (200272): [<ffffffffb0f56121>] finish_task_switch+0x131/0x320 hardirqs last disabled at (200271): [<ffffffffb25c7859>] __schedule+0x129/0xd70 softirqs last enabled at (0): [<ffffffffb0ee093f>] copy_process+0x4df/0x1cc0 softirqs last disabled at (0): [<0000000000000000>] 0x0
------------------------------------------------------------------------
The problem is that irq-work handlers run with interrupts enabled, which means that rcu_preempt_deferred_qs_handler() could be interrupted, and that interrupt handler might contain an RCU read-side critical section, which might invoke rcu_read_unlock_special(). In the strict KCSAN mode of operation used by RCU, this constitutes a data race on the ->defer_qs_iw_pending field.
This commit therefore disables interrupts across the portion of the rcu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pending field. This suffices because this handler is not a fast path.
Signed-off-by: Paul E. McKenney paulmck@kernel.org Reviewed-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Neeraj Upadhyay (AMD) neeraj.upadhyay@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/rcu/tree_plugin.h | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 304e3405e6ec..27ab715569f2 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -623,10 +623,13 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t) */ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp) { + unsigned long flags; struct rcu_data *rdp;
rdp = container_of(iwp, struct rcu_data, defer_qs_iw); + local_irq_save(flags); rdp->defer_qs_iw_pending = false; + local_irq_restore(flags); }
/*