Re: [PATCH 2/2] hazptr: Introduce CONFIG_HAZPTR_DEBUG misuse detection

From: Paul E. McKenney

Date: Thu Jul 09 2026 - 14:57:14 EST

On Thu, Jul 09, 2026 at 02:29:33PM -0400, Mathieu Desnoyers wrote:
> 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:

Thank you, Mathieu!

I am testing this one first to verify the debugging, and then I will
test the patch 1/2 hazptrtorture.c changes to verify the fix.

We are going to need to carefully document the hazard-pointer handling
rules! ;-)

Thanx, Paul

> 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:
> <TASK>
> 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
> </TASK>
> ---[ end trace 0000000000000000 ]---
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---
> 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
>