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;
