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(mtl.efficios.com [216.120.195.104]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4gx3RT0kF0zW9C; Thu, 09 Jul 2026 14:30:13 -0400 (EDT) From: Mathieu Desnoyers To: "Paul E . McKenney" , Boqun Feng Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers Subject: [PATCH 2/2] hazptr: Introduce CONFIG_HAZPTR_DEBUG misuse detection Date: Thu, 9 Jul 2026 14:29:33 -0400 Message-ID: <20260709183009.6814-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260709183009.6814-1-mathieu.desnoyers@efficios.com> References: <20260709183009.6814-1-mathieu.desnoyers@efficios.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Introduce Hazard Pointers debug assert, which detects misuse of hazard pointers, namely failure to detach the hazard pointer from its owner thread before releasing it from a different thread. Prints the following to the console when a failure is detected: Hazard Pointer (addr=000000006885a05f) released on remote task without being detached from task. Acquire: caller=hazptr_torture_read_lock+0x43/0xa0 [hazptrtorture], pid=3727, cpu=1. Release: pid=3725, cpu=139. WARNING: ./include/linux/hazptr.h:225 at hazptr_torture_read_unlock+0x68/0xf0 [hazptrtorture], CPU#139: hazptr_torture_/3725 Modules linked in: hazptrtorture torture nft_masq nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink CPU: 139 UID: 0 PID: 3725 Comm: hazptr_torture_ Not tainted 7.1.0-rc4+ #9 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:hazptr_torture_read_unlock+0x74/0xf0 [hazptrtorture] Code: 74 31 8b 4f 40 4c 8b 43 48 49 c7 c2 22 c9 56 c0 48 c7 c7 3b c9 56 c0 4c 8d 1d b8 32 f1 ff 52 48 89 fa 4c 89 df 50 51 4c 89 d1 <67> 48 0f b9 3a 48 83 c4 18 48 8b 03 48 8d 53 18 48 c7 00 00 00 00 RSP: 0018:ff621a2a479b3de0 EFLAGS: 00010293 RAX: 0000000000000e8d RBX: ff12ea30d3913488 RCX: ffffffffc056c922 RDX: ffffffffc056c93b RSI: ffffffffc0562280 RDI: ffffffffc0562030 RBP: ff621a2a479b3e50 R08: ffffffffc064ee53 R09: 0000000000000e8f R10: ffffffffc056c922 R11: ffffffffc0562030 R12: 0000000000000000 R13: ffffffffc0562280 R14: ff12ea30d3913488 R15: ff621a2a479b3e50 FS: 0000000000000000(0000) GS:ff12ea5011a84000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f39b6e4b010 CR3: 0000000114c6e005 CR4: 0000000000771ef0 PKRU: 55555554 Call Trace: hazptr_torture_reader_tail+0x8e/0x210 [hazptrtorture] hazptr_torture_reader+0x145/0xb30 [hazptrtorture] ? srso_alias_return_thunk+0x5/0xfbef5 ? set_cpus_allowed_ptr+0x36/0x60 ? srso_alias_return_thunk+0x5/0xfbef5 ? srso_alias_return_thunk+0x5/0xfbef5 ? __pfx_hazptr_torture_reader+0x10/0x10 [hazptrtorture] kthread+0xdf/0x120 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x216/0x2d0 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 ---[ end trace 0000000000000000 ]--- Signed-off-by: Mathieu Desnoyers --- include/linux/hazptr.h | 38 ++++++++++++++++++++++++++++++++++++++ kernel/rcu/Kconfig.debug | 9 +++++++++ 2 files changed, 47 insertions(+) diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h index d96414575739..c5aa4fb03b11 100644 --- a/include/linux/hazptr.h +++ b/include/linux/hazptr.h @@ -51,6 +51,11 @@ struct hazptr_ctx { /* Backup slot in case all per-CPU slots are used. */ struct hazptr_backup_slot backup_slot; struct hlist_node preempt_node; +#ifdef CONFIG_HAZPTR_DEBUG + bool detach_task, detach_cpu; /* Whether the ctx has been detached from task/cpu. */ + int acquire_pid, acquire_cpu; /* Note the task and cpu number at acquire. */ + unsigned long acquire_caller; /* Acquire instruction pointer. */ +#endif }; struct hazptr_slot_ctx { @@ -127,6 +132,9 @@ void hazptr_detach_from_task(struct hazptr_ctx *ctx) struct hazptr_slot *slot; guard(preempt)(); +#ifdef CONFIG_HAZPTR_DEBUG + ctx->detach_task = ctx->detach_cpu = true; +#endif slot = ctx->slot; if (unlikely(hazptr_slot_is_backup(ctx, slot))) return; @@ -145,6 +153,9 @@ void hazptr_note_context_switch(void) if (!slot->addr) continue; +#ifdef CONFIG_HAZPTR_DEBUG + item->ctx.ctx->detach_cpu = true; +#endif hazptr_promote_to_backup_slot(item->ctx.ctx, slot); } } @@ -173,6 +184,12 @@ void *hazptr_acquire(struct hazptr_ctx *ctx, void * const *addr_p) percpu_slots = this_cpu_ptr(&hazptr_percpu_slots); slot_item = &percpu_slots->items[0]; slot = &slot_item->slot; +#ifdef CONFIG_HAZPTR_DEBUG + ctx->detach_cpu = ctx->detach_task = false; + ctx->acquire_pid = current->pid; + ctx->acquire_cpu = smp_processor_id(); + ctx->acquire_caller = _THIS_IP_; +#endif if (unlikely(slot->addr)) return __hazptr_acquire(ctx, addr_p); WRITE_ONCE(slot->addr, HAZPTR_WILDCARD); /* Store B */ @@ -196,6 +213,26 @@ void *hazptr_acquire(struct hazptr_ctx *ctx, void * const *addr_p) return addr; } +#ifdef CONFIG_HAZPTR_DEBUG +/* Called with preemption disabled. */ +static inline +void hazptr_release_debug(struct hazptr_ctx *ctx, void *addr) +{ + int pid = current->pid, cpu = smp_processor_id(); + bool warn_remote_cpu = !ctx->detach_cpu && ctx->acquire_cpu != cpu, + warn_remote_task = !ctx->detach_task && ctx->acquire_pid != pid; + + WARN_ONCE(warn_remote_cpu || warn_remote_task, + "Hazard Pointer (addr=%p) released on remote %s without %s. Acquire: caller=%pS, pid=%d, cpu=%d. Release: pid=%d, cpu=%d.", + addr, + warn_remote_task ? "task" : "cpu", + warn_remote_task ? "being detached from task" : "context switch", + (void *) ctx->acquire_caller, ctx->acquire_pid, ctx->acquire_cpu, pid, cpu); +} +#else +static inline void hazptr_release_debug(struct hazptr_ctx *ctx, void *addr) { } +#endif + /* Release the protected hazard pointer from @slot. */ static inline void hazptr_release(struct hazptr_ctx *ctx, void *addr) @@ -205,6 +242,7 @@ void hazptr_release(struct hazptr_ctx *ctx, void *addr) if (!addr) return; guard(preempt)(); + hazptr_release_debug(ctx, addr); slot = ctx->slot; smp_store_release(&slot->addr, NULL); if (unlikely(hazptr_slot_is_backup(ctx, slot))) diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug index 7629c345b0b6..57fec4444a97 100644 --- a/kernel/rcu/Kconfig.debug +++ b/kernel/rcu/Kconfig.debug @@ -264,4 +264,13 @@ config TRIVIAL_PREEMPT_RCU This has no value for production and is only for testing. +config HAZPTR_DEBUG + bool "Provide debugging asserts for Hazard Pointers" + depends on DEBUG_KERNEL + default n + help + This option provides consistency checks for Hazard pointers. + + Say Y here if you want to enable those assert, N otherwise. + endmenu # "RCU Debugging" -- 2.43.0