Re: [PATCH] mm/memory-failure: Use raw_spinlock_t in struct memory_failure_cpu

[Waiman Long <longman@redhat.com>]

On 8/6/24 11:53, Juri Lelli wrote:
> Hi Waimain,
>
> On 06/08/24 10:25, Waiman Long wrote:
>> The memory_failure_cpu structure is a per-cpu structure. Access to its
>> content requires the use of get_cpu_var() to lock in the current CPU
>> and disable preemption. The use of a regular spinlock_t for locking
>> purpose is fine for a non-RT kernel.
>>
>> Since the integration of RT spinlock support into the v5.15 kernel,
>> a spinlock_t in a RT kernel becomes a sleeping lock and taking a
>> sleeping lock in a preemption disabled context is illegal resulting in
>> the following kind of warning.
>>
>>    [12135.732244] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
>>    [12135.732248] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 270076, name: kworker/0:0
>>    [12135.732252] preempt_count: 1, expected: 0
>>    [12135.732255] RCU nest depth: 2, expected: 2
>>      :
>>    [12135.732420] Hardware name: Dell Inc. PowerEdge R640/0HG0J8, BIOS 2.10.2 02/24/2021
>>    [12135.732423] Workqueue: kacpi_notify acpi_os_execute_deferred
>>    [12135.732433] Call Trace:
>>    [12135.732436]  <TASK>
>>    [12135.732450]  dump_stack_lvl+0x57/0x81
>>    [12135.732461]  __might_resched.cold+0xf4/0x12f
>>    [12135.732479]  rt_spin_lock+0x4c/0x100
>>    [12135.732491]  memory_failure_queue+0x40/0xe0
>>    [12135.732503]  ghes_do_memory_failure+0x53/0x390
>>    [12135.732516]  ghes_do_proc.constprop.0+0x229/0x3e0
>>    [12135.732575]  ghes_proc+0xf9/0x1a0
>>    [12135.732591]  ghes_notify_hed+0x6a/0x150
>>    [12135.732602]  notifier_call_chain+0x43/0xb0
>>    [12135.732626]  blocking_notifier_call_chain+0x43/0x60
>>    [12135.732637]  acpi_ev_notify_dispatch+0x47/0x70
>>    [12135.732648]  acpi_os_execute_deferred+0x13/0x20
>>    [12135.732654]  process_one_work+0x41f/0x500
>>    [12135.732695]  worker_thread+0x192/0x360
>>    [12135.732715]  kthread+0x111/0x140
>>    [12135.732733]  ret_from_fork+0x29/0x50
>>    [12135.732779]  </TASK>
>>
>> Fix it by using a raw_spinlock_t for locking instead.
> IIUC this is executed to recover a fault condition already, so maybe
> latencies are of no interest at that point, but I wonder if something
> like
>
> https://elixir.bootlin.com/linux/v6.10.1/source/Documentation/locking/locktypes.rst#L434
>
> would still work and save us from introducing a raw_spinlock?
>
> Or maybe the critical section is anyway tiny and we don't care either?

There are only 2 critical sections that makes use of this lock - 
memory_failure_queue() and memory_failure_work_func().  In 
memory_failure_queue(), there is a kfifo_put() and either 
schedule_work_on() or pr_err(). In memory_failure_work_func(), the 
critical section is just a kfifo_get(). kfifo_get() and kfifo_put() are 
not using loop and their run time, though not very short, shouldn't be 
long. The schedule_work_on() will take its own raw_spinlock_t to do its 
work anyway. So the only call that may have a long runtime is pr_err() 
before the printk rework lands. Fortunately, we can easily take the 
pr_err() call out of the critical section.

As memory_failure_queue() is not a frequently called function and I 
doubt there will be much contention in the lock, I believe it is easier 
to understand to just use raw_spinlock_t than using migrate_disable() 
without using get_cpu_var(). Also if there is hardware issue leading to 
the call to memory_failure_queue(), a bit extra latency due to the use 
of raw_spinlock_t is not the most important concern anyway.

I will post a v2 patch to move pr_err() call out of the lock critical 
section.

Cheers,
Longman