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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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@google.com/
> 
> Cc: byungchul.park@lge.com
> Cc: kernel-team@android.com
> Cc: rcu@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

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;