From: "Luis Claudio R. Goncalves" <lgoncalv@redhat•com>
To: Waiman Long <longman@redhat•com>,
Ada Couprie Diaz <ada.coupriediaz@arm•com>
Cc: Catalin Marinas <catalin.marinas@arm•com>,
Will Deacon <will@kernel•org>,
Mark Rutland <mark.rutland@arm•com>,
Sebastian Andrzej Siewior <bigeasy@linutronix•de>,
Clark Williams <clrkwllms@kernel•org>,
Steven Rostedt <rostedt@goodmis•org>,
linux-arm-kernel@lists•infradead.org,
linux-kernel@vger•kernel.org, linux-rt-devel@lists•linux.dev
Subject: Re: [PATCH] arm64/entry: Don't disable preemption in debug_exception_enter() with RT kernel
Date: Tue, 19 May 2026 21:23:41 -0300 [thread overview]
Message-ID: <agz_DYGnUsimJ3xL@redhat.com> (raw)
In-Reply-To: <20260519222524.886454-1-longman@redhat.com>
On Tue, May 19, 2026 at 06:25:24PM -0400, Waiman Long wrote:
> Commit d8bb6718c4db ("arm64: Make debug exception handlers visible from
> RCU") introduces debug_exception_enter() and debug_exception_exit()
> where preemption is explicitly disabled. With a PREEMPT_RT debug kernel,
> the following bug report can happen.
>
> BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 15255, name: gdb_app
> preempt_count: 1, expected: 0
> RCU nest depth: 0, expected: 0
> 1 lock held by gdb_app/15255:
> #0: ffff10007f41b7d8 (&sighand->siglock){..}-{3:3}, at: force_sig_info_to_task+0x34/0x130
> Preemption disabled at:
> [<ffff800080081ea8>] debug_exception_enter+0x18/0x70
> :
> Call trace:
> dump_backtrace+0xac/0x130
> show_stack+0x1c/0x24
> dump_stack_lvl+0xa0/0xe0
> dump_stack+0x14/0x2c
> __might_resched+0x178/0x230
> rt_spin_lock+0x58/0x120
> force_sig_info_to_task+0x34/0x130
> force_sig_fault+0x58/0x80
> arm64_force_sig_fault+0x44/0x70
> send_user_sigtrap+0x5c/0xa0
> brk_handler+0x38/0x5c
> do_debug_exception+0x78/0x110
> el0_dbg+0x50/0x1e0
> el0t_64_sync_handler+0x114/0x150
> el0t_64_sync+0x17c/0x180
>
> Fix that by blocking the preempt_disable()/preempt_enable_no_resched()
> calls when CONFIG_PREEMPT_RT is enabled.
Hi Waiman!
Last year Ada Couprie Diaz wrote a patcheseries that enhanced greatly the
ARM64 debug exception code. In the cover letter there is a discussion about
the effect of the patches on RT[0] (look for PREEMPT_RT), explaining that
there are a few remaining known bugs and briefly discussing the best way to
fix then. There is also a discussion[1] about the specific issue you reported.
I took the liberty of adding Ada to the thread.
Best regards,
Luis
[0] https://lore.kernel.org/all/20250707114109.35672-1-ada.coupriediaz@arm.com/
[1] https://lore.kernel.org/linux-arm-kernel/e86c5c3a-6666-46a7-b7ec-e803212a81a1@arm.com/
> Signed-off-by: Waiman Long <longman@redhat•com>
> ---
> arch/arm64/kernel/entry-common.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index c7a23f7c2212..191441b22b7c 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -290,15 +290,17 @@ static __always_inline void fpsimd_syscall_exit(void)
> }
>
> /*
> - * In debug exception context, we explicitly disable preemption despite
> - * having interrupts disabled.
> + * In debug exception context, we explicitly disable preemption except for
> + * PREEMPT_RT kernel as rt_spin_lock() can be called.
> + *
> * This serves two purposes: it makes it much less likely that we would
> * accidentally schedule in exception context and it will force a warning
> * if we somehow manage to schedule by accident.
> */
> static void debug_exception_enter(struct pt_regs *regs)
> {
> - preempt_disable();
> + if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> + preempt_disable();
>
> /* This code is a bit fragile. Test it. */
> RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
> @@ -307,7 +309,8 @@ NOKPROBE_SYMBOL(debug_exception_enter);
>
> static void debug_exception_exit(struct pt_regs *regs)
> {
> - preempt_enable_no_resched();
> + if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> + preempt_enable_no_resched();
> }
> NOKPROBE_SYMBOL(debug_exception_exit);
>
> --
> 2.54.0
>
>
---end quoted text---
next prev parent reply other threads:[~2026-05-20 0:26 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-19 22:25 [PATCH] arm64/entry: Don't disable preemption in debug_exception_enter() with RT kernel Waiman Long
2026-05-20 0:23 ` Luis Claudio R. Goncalves [this message]
2026-05-20 6:19 ` Sebastian Andrzej Siewior
2026-05-20 11:23 ` Luis Claudio R. Goncalves
2026-05-20 21:01 ` Waiman Long
2026-05-28 14:51 ` Sebastian Andrzej Siewior
2026-05-28 15:35 ` Sebastian Andrzej Siewior
2026-05-30 0:51 ` Luis Claudio R. Goncalves
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