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SubjectRe: [PATCH 2/2] locktorture: call percpu_free_rwsem() to do percpu-rwsem cleanup
On Thu, Sep 17, 2020 at 09:59:10PM +0800, Hou Tao wrote:
> When do percpu-rwsem writer lock torture, the RCU callback
> rcu_sync_func() may still be pending after locktorture module
> is removed, and it will lead to the following Oops:
>
> BUG: unable to handle page fault for address: ffffffffc00eb920
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 6500a067 P4D 6500a067 PUD 6500c067 PMD 13a36c067 PTE 800000013691c163
> Oops: 0000 [#1] PREEMPT SMP
> CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.9.0-rc5+ #4
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
> RIP: 0010:rcu_cblist_dequeue+0x12/0x30
> Call Trace:
> <IRQ>
> rcu_core+0x1b1/0x860
> __do_softirq+0xfe/0x326
> asm_call_on_stack+0x12/0x20
> </IRQ>
> do_softirq_own_stack+0x5f/0x80
> irq_exit_rcu+0xaf/0xc0
> sysvec_apic_timer_interrupt+0x2e/0xb0
> asm_sysvec_apic_timer_interrupt+0x12/0x20
>
> Fix it by adding an exit hook in lock_torture_ops and
> use it to call percpu_free_rwsem() for percpu rwsem torture
> before the module is removed, so we can ensure rcu_sync_func()
> completes before module exits.
>
> Also needs to call exit hook if lock_torture_init() fails half-way,
> so use ctx->cur_ops != NULL to signal that init hook has been called.

Good catch, but please see below for comments and questions.

> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
> kernel/locking/locktorture.c | 28 ++++++++++++++++++++++------
> 1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
> index bebdf98e6cd78..e91033e9b6f95 100644
> --- a/kernel/locking/locktorture.c
> +++ b/kernel/locking/locktorture.c
> @@ -74,6 +74,7 @@ static void lock_torture_cleanup(void);
> */
> struct lock_torture_ops {
> void (*init)(void);
> + void (*exit)(void);

This is fine, but why not also add a flag to the lock_torture_cxt
structure that is set when the ->init() function is called? Perhaps
something like this in lock_torture_init():

if (cxt.cur_ops->init) {
cxt.cur_ops->init();
cxt.initcalled = true;
}

> int (*writelock)(void);
> void (*write_delay)(struct torture_random_state *trsp);
> void (*task_boost)(struct torture_random_state *trsp);
> @@ -571,6 +572,11 @@ void torture_percpu_rwsem_init(void)
> BUG_ON(percpu_init_rwsem(&pcpu_rwsem));
> }
>
> +static void torture_percpu_rwsem_exit(void)
> +{
> + percpu_free_rwsem(&pcpu_rwsem);
> +}
> +
> static int torture_percpu_rwsem_down_write(void) __acquires(pcpu_rwsem)
> {
> percpu_down_write(&pcpu_rwsem);
> @@ -595,6 +601,7 @@ static void torture_percpu_rwsem_up_read(void) __releases(pcpu_rwsem)
>
> static struct lock_torture_ops percpu_rwsem_lock_ops = {
> .init = torture_percpu_rwsem_init,
> + .exit = torture_percpu_rwsem_exit,
> .writelock = torture_percpu_rwsem_down_write,
> .write_delay = torture_rwsem_write_delay,
> .task_boost = torture_boost_dummy,
> @@ -786,9 +793,10 @@ static void lock_torture_cleanup(void)
>
> /*
> * Indicates early cleanup, meaning that the test has not run,
> - * such as when passing bogus args when loading the module. As
> - * such, only perform the underlying torture-specific cleanups,
> - * and avoid anything related to locktorture.
> + * such as when passing bogus args when loading the module.
> + * However cxt->cur_ops.init() may have been invoked, so beside
> + * perform the underlying torture-specific cleanups, cur_ops.exit()
> + * will be invoked if needed.
> */
> if (!cxt.lwsa && !cxt.lrsa)
> goto end;
> @@ -828,6 +836,12 @@ static void lock_torture_cleanup(void)
> cxt.lrsa = NULL;
>
> end:
> + /* If init() has been called, then do exit() accordingly */
> + if (cxt.cur_ops) {
> + if (cxt.cur_ops->exit)
> + cxt.cur_ops->exit();
> + cxt.cur_ops = NULL;
> + }

The above can then be:

if (cxt.initcalled && cxt.cur_ops->exit)
cxt.cur_ops->exit();

Maybe you also need to clear cxt.initcalled at this point, but I don't
immediately see why that would be needed.

> torture_cleanup_end();
> }
>
> @@ -835,6 +849,7 @@ static int __init lock_torture_init(void)
> {
> int i, j;
> int firsterr = 0;
> + struct lock_torture_ops *cur_ops;

And then you don't need this extra pointer. Not that this pointer is bad
in and of itself, but using (!cxt.cur_ops) to indicate that the ->init()
function has not been called is an accident waiting to happen.

And the changes below are no longer needed.

Or am I missing something subtle?

Thanx, Paul

> static struct lock_torture_ops *torture_ops[] = {
> &lock_busted_ops,
> &spin_lock_ops, &spin_lock_irq_ops,
> @@ -853,8 +868,8 @@ static int __init lock_torture_init(void)
>
> /* Process args and tell the world that the torturer is on the job. */
> for (i = 0; i < ARRAY_SIZE(torture_ops); i++) {
> - cxt.cur_ops = torture_ops[i];
> - if (strcmp(torture_type, cxt.cur_ops->name) == 0)
> + cur_ops = torture_ops[i];
> + if (strcmp(torture_type, cur_ops->name) == 0)
> break;
> }
> if (i == ARRAY_SIZE(torture_ops)) {
> @@ -869,12 +884,13 @@ static int __init lock_torture_init(void)
> }
>
> if (nwriters_stress == 0 &&
> - (!cxt.cur_ops->readlock || nreaders_stress == 0)) {
> + (!cur_ops->readlock || nreaders_stress == 0)) {
> pr_alert("lock-torture: must run at least one locking thread\n");
> firsterr = -EINVAL;
> goto unwind;
> }
>
> + cxt.cur_ops = cur_ops;
> if (cxt.cur_ops->init)
> cxt.cur_ops->init();
>
> --
> 2.25.0.4.g0ad7144999
>

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