Re: [PATCH 1/2] rcutorture: Avoid corner-case #DE with nsynctypes check

[Paul E. McKenney]

On Mon, Mar 07, 2022 at 02:46:55PM -0800, David Vernet wrote:
> The rcutorture module is used to run torture tests that validate RCU.
> rcutorture takes a variety of module parameters that configure the
> functionality of the test. Amongst these parameters are the types of
> synchronization mechanisms that the rcu_torture_writer and
> rcu_torture_fakewriter tasks may use, and the torture_type of the run which
> determines what read and sync operations are used by the various writer and
> reader tasks that run throughout the test.
> 
> When the module is configured to only use sync types for which the
> specified torture_type does not implement the necessary operations, we can
> end up in a state where nsynctypes is 0. This is not an erroneous state,
> but it currently crashes the kernel with a #DE due to nsynctypes being used
> with a modulo operator in rcu_torture_fakewriter().
> 
> Here is an example of such a #DE:
> 
> $ insmod ./rcutorture.ko gp_cond=1 gp_cond_exp=0 gp_exp=0 gp_poll_exp=0
> gp_normal=0 gp_poll=0 gp_poll_exp=0 verbose=9999 torture_type=trivial
> 
> ...
> 
> [ 8536.525096] divide error: 0000 [#1] PREEMPT SMP PTI
> [ 8536.525101] CPU: 30 PID: 392138 Comm: rcu_torture_fak Kdump: loaded Tainted: G S                5.17.0-rc1-00179-gc8c42c80febd #24
> [ 8536.525105] Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A23 12/08/2020
> [ 8536.525106] RIP: 0010:rcu_torture_fakewriter+0xf1/0x2d0 [rcutorture]
> [ 8536.525121] Code: 00 31 d2 8d 0c f5 00 00 00 00 48 63 c9 48 f7 f1 48 85 d2 0f 84 79 ff ff ff 48 89 e7 e8 78 78 01 00 48 63 0d 29 ca 00 00 31 d2 <48> f7 f1 8b 04 95 00 05 4e a0 83 f8 06 0f 84 ad 00 00 00 7f 1f 83
> [ 8536.525124] RSP: 0018:ffffc9000777fef0 EFLAGS: 00010246
> [ 8536.525127] RAX: 00000000223d006e RBX: cccccccccccccccd RCX: 0000000000000000
> [ 8536.525130] RDX: 0000000000000000 RSI: ffffffff824315b9 RDI: ffffc9000777fef0
> [ 8536.525132] RBP: ffffc9000487bb30 R08: 0000000000000002 R09: 000000000002a580
> [ 8536.525134] R10: ffffffff82c5f920 R11: 0000000000000000 R12: ffff8881a2c35d00
> [ 8536.525136] R13: ffff8881540c8d00 R14: ffffffffa04d39d0 R15: 0000000000000000
> [ 8536.525137] FS:  0000000000000000(0000) GS:ffff88903ff80000(0000) knlGS:0000000000000000
> [ 8536.525140] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 8536.525142] CR2: 00007f839f022000 CR3: 0000000002c0a006 CR4: 00000000007706e0
> [ 8536.525144] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 8536.525145] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 8536.525147] PKRU: 55555554
> [ 8536.525148] Call Trace:
> [ 8536.525150]  <TASK>
> [ 8536.525153]  kthread+0xe8/0x110
> [ 8536.525161]  ? kthread_complete_and_exit+0x20/0x20
> [ 8536.525167]  ret_from_fork+0x22/0x30
> [ 8536.525174]  </TASK>
> 
> The solution is to gracefully handle the case of nsynctypes being 0 in
> rcu_torture_fakewriter() by not performing any work. This is already being
> done in rcu_torture_writer(), though there is a missing return on that path
> which will be fixed in a subsequent patch.
> 
> Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>

Applied for further review and testing, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/rcutorture.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 57faf4cb9cd2..7fdff1bfed79 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1371,6 +1371,17 @@ rcu_torture_fakewriter(void *arg)
>  	VERBOSE_TOROUT_STRING("rcu_torture_fakewriter task started");
>  	set_user_nice(current, MAX_NICE);
>  
> +	if (WARN_ONCE(nsynctypes == 0,
> +		      "%s: No update-side primitives.\n", __func__)) {
> +		/*
> +		 * No updates primitives, so don't try updating.
> +		 * The resulting test won't be testing much, hence the
> +		 * above WARN_ONCE().
> +		 */
> +		torture_kthread_stopping("rcu_torture_fakewriter");
> +		return 0;
> +	}
> +
>  	do {
>  		torture_hrtimeout_jiffies(torture_random(&rand) % 10, &rand);
>  		if (cur_ops->cb_barrier != NULL &&
> -- 
> 2.30.2
>