[linux-yocto] [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG

[zhe.he at windriver.com]

From: Sebastian Andrzej Siewior <bigeasy at linutronix.de>

commit 97731753d44d5efcb95b994dc952c0e8195b3e96 upstream

This patch reverts commit c7f26ccfb2c3 ("mm/vmstat.c: fix
vmstat_update() preemption BUG").
Steven saw a "using smp_processor_id() in preemptible" message and
added a preempt_disable() section around it to keep it quiet. This is
not the right thing to do it does not fix the real problem.

vmstat_update() is invoked by a kworker on a specific CPU. This worker
it bound to this CPU. The name of the worker was "kworker/1:1" so it
should have been a worker which was bound to CPU1. A worker which can
run on any CPU would have a `u' before the first digit.

smp_processor_id() can be used in a preempt-enabled region as long as
the task is bound to a single CPU which is the case here. If it could
run on an arbitrary CPU then this is the problem we have an should seek
to resolve.
Not only this smp_processor_id() must not be migrated to another CPU but
also refresh_cpu_vm_stats() which might access wrong per-CPU variables.
Not to mention that other code relies on the fact that such a worker
runs on one specific CPU only.

Therefore I revert that commit and we should look instead what broke the
affinity mask of the kworker.

Cc: Steven J. Hill <steven.hill at cavium.com>
Cc: Tejun Heo <htejun at gmail.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt at goodmis.org>

Backport this to fix following serious crash for preempt-rt kernel

BUG: scheduling while atomic: kworker/3:1/91/0x00000002
Preemption disabled at:
[<ffffffff8437c37d>] vmstat_update+0x2d/0xb0
CPU: 3 PID: 91 Comm: kworker/3:1 Not tainted 4.14.48-rt29-yocto-preempt-rt #1
Hardware name: Intel Corporation Broadwell Client platform/Basking Ridge, BIOS BDW-E2R1.86C.0118.R01.1503110618 03/11/2015
Workqueue: mm_percpu_wq vmstat_update
Call Trace:
 dump_stack+0x4f/0x6b
 ? vmstat_update+0x2d/0xb0
 __schedule_bug.cold.24+0x7d/0x9a
 __schedule+0x45a/0x6c0
 ? task_blocks_on_rt_mutex+0x173/0x300
 schedule+0x3d/0xd0
 rt_spin_lock_slowlock_locked+0x118/0x2a0
 rt_spin_lock_slowlock+0x48/0x60
 rt_spin_lock+0x3f/0x50
 queue_delayed_work_on+0x63/0x100
 vmstat_update+0x6e/0xb0
 process_one_work+0x1c3/0x430
 worker_thread+0x32/0x440
 kthread+0x127/0x140
 ? process_one_work+0x430/0x430
 ? kthread_create_on_node+0x40/0x40
 ret_from_fork+0x35/0x40

Signed-off-by: He Zhe <zhe.he at windriver.com>
---
 mm/vmstat.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 84fd6eb..0d17b8f 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1782,11 +1782,9 @@ static void vmstat_update(struct work_struct *w)
 		 * to occur in the future. Keep on running the
 		 * update worker thread.
 		 */
-		preempt_disable();
 		queue_delayed_work_on(smp_processor_id(), mm_percpu_wq,
 				this_cpu_ptr(&vmstat_work),
 				round_jiffies_relative(sysctl_stat_interval));
-		preempt_enable();
 	}
 }
 
-- 
2.7.4