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

[Bruce Ashfield]

Which version/branches is this for ?

Given the author, I'm assuming it is for the -rt branches and also
for 4.12 .. but I'd like a confirmation of that.

Bruce

On 2018-07-20 4:13 AM, zhe.he at windriver.com wrote:
> 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();
>   	}
>   }
>   
>