linux-kernel - Re: [RESEND PATCH] riscv: Fixup boot failure when CONFIG_DEBUG_RT

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Message-ID: <7e93d7a9-caa8-40c4-997a-d96ad94dbe94@ghiti.fr>
Date: Wed, 4 Dec 2024 09:27:00 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: guoren@...nel.org, paul.walmsley@...ive.com, palmer@...belt.com,
 bjorn@...osinc.com, conor@...nel.org, leobras@...hat.com,
 peterz@...radead.org, parri.andrea@...il.com, will@...nel.org,
 longman@...hat.com, boqun.feng@...il.com, arnd@...db.de,
 alexghiti@...osinc.com
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [RESEND PATCH] riscv: Fixup boot failure when
 CONFIG_DEBUG_RT_MUTEXES=y

Hi Guo,

On 30/11/2024 16:33, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
>
> When CONFIG_DEBUG_RT_MUTEXES=y, mutex_lock->rt_mutex_try_acquire
> would change from rt_mutex_cmpxchg_acquire to
> rt_mutex_slowtrylock():
> 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
> 	ret = __rt_mutex_slowtrylock(lock);
> 	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>
> Because queued_spin_#ops to ticket_#ops is changed one by one by
> jump_label, raw_spin_lock/unlock would cause a deadlock during the
> changing.
>
> That means in arch/riscv/kernel/jump_label.c:
> 1.
> arch_jump_label_transform_queue() ->
> mutex_lock(&text_mutex); +-> raw_spin_lock  -> queued_spin_lock
> 			 |-> raw_spin_unlock -> queued_spin_unlock
> patch_insn_write -> change the raw_spin_lock to ticket_lock
> mutex_unlock(&text_mutex);
> ...
>
> 2. /* Dirty the lock value */
> arch_jump_label_transform_queue() ->
> mutex_lock(&text_mutex); +-> raw_spin_lock -> *ticket_lock*
>                           |-> raw_spin_unlock -> *queued_spin_unlock*
> 			  /* BUG: ticket_lock with queued_spin_unlock */
> patch_insn_write  ->  change the raw_spin_unlock to ticket_unlock
> mutex_unlock(&text_mutex);
> ...
>
> 3. /* Dead lock */
> arch_jump_label_transform_queue() ->
> mutex_lock(&text_mutex); +-> raw_spin_lock -> ticket_lock /* deadlock! */
>                           |-> raw_spin_unlock -> ticket_unlock
> patch_insn_write -> change other raw_spin_#op -> ticket_#op
> mutex_unlock(&text_mutex);
>
> So, the solution is to disable mutex usage of
> arch_jump_label_transform_queue() during early_boot_irqs_disabled, just
> like we have done for stop_machine.
>
> Reported-by: Conor Dooley <conor@...nel.org>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Fixes: ab83647fadae ("riscv: Add qspinlock support")
> Link: https://lore.kernel.org/linux-riscv/CAJF2gTQwYTGinBmCSgVUoPv0_q4EPt_+WiyfUA1HViAKgUzxAg@mail.gmail.com/T/#mf488e6347817fca03bb93a7d34df33d8615b3775
> Cc: Palmer Dabbelt <palmer@...belt.com>
> Cc: Alexandre Ghiti <alexghiti@...osinc.com>
> ---
>   arch/riscv/kernel/jump_label.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/jump_label.c b/arch/riscv/kernel/jump_label.c
> index 6eee6f736f68..654ed159c830 100644
> --- a/arch/riscv/kernel/jump_label.c
> +++ b/arch/riscv/kernel/jump_label.c
> @@ -36,9 +36,15 @@ bool arch_jump_label_transform_queue(struct jump_entry *entry,
>   		insn = RISCV_INSN_NOP;
>   	}
>   
> -	mutex_lock(&text_mutex);
> -	patch_insn_write(addr, &insn, sizeof(insn));
> -	mutex_unlock(&text_mutex);
> +	if (early_boot_irqs_disabled) {
> +		riscv_patch_in_stop_machine = 1;
> +		patch_insn_write(addr, &insn, sizeof(insn));
> +		riscv_patch_in_stop_machine = 0;
> +	} else {
> +		mutex_lock(&text_mutex);
> +		patch_insn_write(addr, &insn, sizeof(insn));
> +		mutex_unlock(&text_mutex);
> +	}
>   
>   	return true;
>   }


Sorry for the late answer, I've been sick lately!

Thank you very much for looking into this and finding this not-so-bad 
solution! I remind everyone that this is a temporary solution until we 
can use an alternative instead of a static key.

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>
Tested-by: Alexandre Ghiti <alexghiti@...osinc.com>

The revert is still on the table IMO, let's Palmer decide.

Thank you again Guo, really appreciate you took the time to find this 
solution!

Alex