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<70e3482b-3181-446c-b3bd-db590008123e@nvidia.com> Date: Wed, 7 May 2025 12:52:04 -0400 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] rcu/nocb: Add Safe checks for access offloaded rdp To: Frederic Weisbecker Cc: Zqiang , paulmck@kernel.org, neeraj.upadhyay@kernel.org, joel@joelfernandes.org, urezki@gmail.com, boqun.feng@gmail.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250507112605.20910-1-qiang.zhang1211@gmail.com> <20250507112605.20910-2-qiang.zhang1211@gmail.com> Content-Language: en-US From: Joel Fernandes In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MN2PR06CA0013.namprd06.prod.outlook.com (2603:10b6:208:23d::18) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 May 2025 16:52:07.6555 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: vHDOSAphqyrgosP3nu1xY1UchrYHecWwa7NpKNpfcSG7xR6VuOXO8oJy4ahjOijqSYVXkOQ7d212CMyDBSccIg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM3PR12MB9389 On 5/7/2025 12:31 PM, Frederic Weisbecker wrote: > Le Wed, May 07, 2025 at 12:06:29PM -0400, Joel Fernandes a écrit : >> >> >> On 5/7/2025 7:26 AM, Zqiang wrote: >>> For built with CONFIG_PROVE_RCU=y and CONFIG_PREEMPT_RT=y kernels, >>> Disable BH does not change the SOFTIRQ corresponding bits in >>> preempt_count(), but change current->softirq_disable_cnt, this >>> resulted in the following splat: >>> >>> WARNING: suspicious RCU usage >>> kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state! >>> stack backtrace: >>> CPU: 0 UID: 0 PID: 22 Comm: rcuc/0 >>> Call Trace: >>> [ 0.407907] >>> [ 0.407910] dump_stack_lvl+0xbb/0xd0 >>> [ 0.407917] dump_stack+0x14/0x20 >>> [ 0.407920] lockdep_rcu_suspicious+0x133/0x210 >>> [ 0.407932] rcu_rdp_is_offloaded+0x1c3/0x270 >>> [ 0.407939] rcu_core+0x471/0x900 >>> [ 0.407942] ? lockdep_hardirqs_on+0xd5/0x160 >>> [ 0.407954] rcu_cpu_kthread+0x25f/0x870 >>> [ 0.407959] ? __pfx_rcu_cpu_kthread+0x10/0x10 >>> [ 0.407966] smpboot_thread_fn+0x34c/0xa50 >>> [ 0.407970] ? trace_preempt_on+0x54/0x120 >>> [ 0.407977] ? __pfx_smpboot_thread_fn+0x10/0x10 >>> [ 0.407982] kthread+0x40e/0x840 >>> [ 0.407990] ? __pfx_kthread+0x10/0x10 >>> [ 0.407994] ? rt_spin_unlock+0x4e/0xb0 >>> [ 0.407997] ? rt_spin_unlock+0x4e/0xb0 >>> [ 0.408000] ? __pfx_kthread+0x10/0x10 >>> [ 0.408006] ? __pfx_kthread+0x10/0x10 >>> [ 0.408011] ret_from_fork+0x40/0x70 >>> [ 0.408013] ? __pfx_kthread+0x10/0x10 >>> [ 0.408018] ret_from_fork_asm+0x1a/0x30 >>> [ 0.408042] >>> >>> Currently, triggering an rdp offloaded state change need the >>> corresponding rdp's CPU goes offline, and at this time the rcuc >>> kthreads has already in parking state. this means the corresponding >>> rcuc kthreads can safely read offloaded state of rdp while it's >>> corresponding cpu is online. >>> >>> This commit therefore add softirq_count() check for >>> Preempt-RT kernels. >>> >>> Suggested-by: Joel Fernandes >>> Signed-off-by: Zqiang >>> --- >>> kernel/rcu/tree_plugin.h | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h >>> index 003e549f6514..a91b2322a0cd 100644 >>> --- a/kernel/rcu/tree_plugin.h >>> +++ b/kernel/rcu/tree_plugin.h >>> @@ -29,7 +29,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) >>> (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) || >>> lockdep_is_held(&rdp->nocb_lock) || >>> lockdep_is_held(&rcu_state.nocb_mutex) || >>> - (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) && >>> + ((!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count()) && >>> rdp == this_cpu_ptr(&rcu_data)) || >> This looks good to me. Frederic told me he'll further review and give final >> green signal. Then I'll pull this particular one. >> >> One thing I was wondering -- it would be really nice if preemptible() itself >> checked for softirq_count() by default. Or adding something like a >> really_preemptible() which checks for both CONFIG_PREEMPT_COUNT and >> softirq_count() along with preemptible(). I feel like this always comes back to >> bite us in different ways, and not knowing atomicity complicates various code paths. >> >> Maybe a summer holidays project? ;) > > I thought about that too but I think this is semantically incorrect. > In PREEMPT_RT, softirqs are actually preemptible. You are right, for this usecase it may not be that helpful. However, I do find it odd that the caller has to check CONFIG_PREEMPT_COUNT before calling preemptible() above, that should be implemented in the API itself. I was more broadly referring to the recurring problem of "am I in atomic" context or "can I be preempted" or "can I sleep", if so do this otherwise do something else. For instance calling GFP_KERNEL vs GFP_ATOMIC in the memory allocator. Though sleeping and preemption are perhaps separate concerns. We ran into this during the kfree_rcu() early days as well. IIRC we have perhaps too many APIs that do similar things like in_atomic() which confuses everyone (I don't fully know the current state of all such APIs). Add PREEMPT_RT to the mix, and now we have more complications. :-/ thanks, - Joel