Re: [PATCH v2 2/4] rcutree: Add checks for dynticks counters in rcu_is_cpu_rrupt_from_idle

From: Paul E. McKenney
Date: Wed Mar 27 2019 - 10:58:11 EST

On Tue, Mar 26, 2019 at 03:24:09PM -0400, Joel Fernandes (Google) wrote:
> In the future we would like to combine the dynticks and dynticks_nesting
> counters thus leading to simplifying the code. At the moment we cannot
> do that due to concerns about usermode upcalls appearing to RCU as half
> of an interrupt. Byungchul tried to do it in [1] but the
> "half-interrupt" concern was raised. It is half because, what RCU
> expects is rcu_irq_enter() and rcu_irq_exit() pairs when the usermode
> exception happens. However, only rcu_irq_enter() is observed. This
> concern may not be valid anymore, but at least it used to be the case.
>
> Out of abundance of caution, Paul added warnings [2] in the RCU code
> which if not fired by 2021 may allow us to assume that such
> half-interrupt scenario cannot happen any more, which can lead to
> simplification of this code.
>
> Summary of the changes are the following:
>
> (1) In preparation for this combination of counters in the future, we
> first need to first be sure that rcu_rrupt_from_idle cannot be called
> from anywhere but a hard-interrupt because previously, the comments
> suggested otherwise so let us be sure. We discussed this here [3]. We
> use the services of lockdep to accomplish this.
>
> (2) Further rcu_rrupt_from_idle() is not explicit about how it is using
> the counters which can lead to weird future bugs. This patch therefore
> makes it more explicit about the specific counter values being tested
>
> (3) Lastly, we check for counter underflows just to be sure these are
> not happening, because the previous code in rcu_rrupt_from_idle() was
> allowing the case where the counters can underflow, and the function
> would still return true. Now we are checking for specific values so let
> us be confident by additional checking, that such underflows don't
> happen. Any case, if they do, we should fix them and the screaming
> warning is appropriate. All these checks checks are NOOPs if PROVE_RCU
> and PROVE_LOCKING are disabled.
>
> [1] https://lore.kernel.org/patchwork/patch/952349/
> [2] Commit e11ec65cc8d6 ("rcu: Add warning to detect half-interrupts")
> [3] https://lore.kernel.org/lkml/20190312150514.GB249405@xxxxxxxxxx/
>
> Cc: byungchul.park@xxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> Cc: rcu@xxxxxxxxxxxxxxx
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Color me stupid:

[ 48.845724] ------------[ cut here ]------------
[ 48.846619] Not in hardirq as expected
[ 48.847322] WARNING: CPU: 5 PID: 34 at /home/git/linux-2.6-tip/kernel/rcu/tree.c:388 rcu_is_cpu_rrupt_from_idle+0xea/0x110
[ 48.849302] Modules linked in:
[ 48.849869] CPU: 5 PID: 34 Comm: cpuhp/5 Not tainted 5.1.0-rc1+ #1
[ 48.850985] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[ 48.852436] RIP: 0010:rcu_is_cpu_rrupt_from_idle+0xea/0x110
[ 48.853455] Code: 85 c0 0f 85 59 ff ff ff 80 3d 33 55 68 01 00 0f 85 4c ff ff ff 48 c7 c7 48 d8 cc 8e 31 c0 c6 05 1d 55 68 01 01 e8 66 54 f8 ff <0f> 0b e9 30 ff ff ff 65 48 8b 05 df 58 54 72 48 85 c0 0f 94 c0 0f
[ 48.856783] RSP: 0000:ffffbc46802dfdc0 EFLAGS: 00010082
[ 48.857735] RAX: 000000000000001a RBX: 0000000000022b80 RCX: 0000000000000000
[ 48.859028] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8dac906c
[ 48.860313] RBP: ffffbc46802dfe20 R08: 0000000000000001 R09: 0000000000000001
[ 48.861607] R10: 000000007d53d16d R11: ffffbc46802dfb48 R12: ffff9e7d7eb62b80
[ 48.862898] R13: 0000000000000005 R14: ffffffff8dae2ac0 R15: 00000000000000c9
[ 48.864191] FS: 0000000000000000(0000) GS:ffff9e7d7eb40000(0000) knlGS:0000000000000000
[ 48.865663] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 48.866702] CR2: 0000000000000000 CR3: 0000000021022000 CR4: 00000000000006e0
[ 48.867993] Call Trace:
[ 48.868450] rcu_exp_handler+0x35/0x90
[ 48.869147] generic_exec_single+0xab/0x100
[ 48.869918] ? rcu_barrier+0x240/0x240
[ 48.870607] smp_call_function_single+0x8e/0xd0
[ 48.871441] rcutree_online_cpu+0x80/0x90
[ 48.872181] cpuhp_invoke_callback+0xb5/0x890
[ 48.872979] cpuhp_thread_fun+0x172/0x210
[ 48.873722] ? cpuhp_thread_fun+0x2a/0x210
[ 48.874474] smpboot_thread_fn+0x10d/0x160
[ 48.875224] kthread+0xf3/0x130
[ 48.875804] ? sort_range+0x20/0x20
[ 48.876446] ? kthread_cancel_delayed_work_sync+0x10/0x10
[ 48.877445] ret_from_fork+0x3a/0x50
[ 48.878124] irq event stamp: 734
[ 48.878717] hardirqs last enabled at (733): [<ffffffff8e4f332d>] _raw_spin_unlock_irqrestore+0x2d/0x40
[ 48.880402] hardirqs last disabled at (734): [<ffffffff8db0110a>] generic_exec_single+0x9a/0x100
[ 48.881986] softirqs last enabled at (0): [<ffffffff8da5feaf>] copy_process.part.56+0x61f/0x2110
[ 48.883540] softirqs last disabled at (0): [<0000000000000000>] (null)
[ 48.884840] ---[ end trace 00b4c1d2f816f4ed ]---

If a CPU invokes generic_exec_single() on itself, the "IPI handler" will
be invoked directly, triggering your new lockdep check. Which is a bit
wasteful. My thought is to add code to sync_rcu_exp_select_node_cpus()
to check the CPU with preemption disabled, avoiding the call to
smp_call_function_single() in that case.

I have queued all four of your patches, and am trying the fix to
the caller of smp_call_function_single() shown below. Thoughts?

Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 9c990df880d1..51d61028abf1 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -384,7 +384,13 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
mask_ofl_test |= mask;
continue;
}
+ preempt_disable();
+ if (smp_processor_id() == cpu) {
+ preempt_enable();
+ continue;
+ }
ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
+ preempt_enable();
if (!ret) {
mask_ofl_ipi &= ~mask;
continue;