linux-kernel - Re: [PATCH 4/7] sempahore: add a helper for a concurrency limiter

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Message-ID: <20230331041154.GC12892@google.com>
Date:   Fri, 31 Mar 2023 13:11:54 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        david@...hat.com, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, petr.pavlu@...e.com,
        prarit@...hat.com, gregkh@...uxfoundation.org, rafael@...nel.org,
        christophe.leroy@...roup.eu, tglx@...utronix.de, song@...nel.org,
        rppt@...nel.org, willy@...radead.org, vbabka@...e.cz,
        mhocko@...e.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH 4/7] sempahore: add a helper for a concurrency limiter

On (23/03/30 20:45), Luis Chamberlain wrote:
[..]
> -static DEFINE_SEMAPHORE(console_sem);
> +static DEFINE_MUTEX(console_sem);
>  HLIST_HEAD(console_list);
>  EXPORT_SYMBOL_GPL(console_list);
>  DEFINE_STATIC_SRCU(console_srcu);
> @@ -309,7 +309,7 @@ EXPORT_SYMBOL(console_srcu_read_unlock);
>   * macros instead of functions so that _RET_IP_ contains useful information.
>   */
>  #define down_console_sem() do { \
> -	down(&console_sem);\
> +	mutex_lock(&console_sem);\
>  	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
>  } while (0)
>  
> @@ -324,7 +324,7 @@ static int __down_trylock_console_sem(unsigned long ip)
>  	 * deadlock in printk()->down_trylock_console_sem() otherwise.
>  	 */
>  	printk_safe_enter_irqsave(flags);
> -	lock_failed = down_trylock(&console_sem);
> +	lock_failed = !mutex_trylock(&console_sem);
>  	printk_safe_exit_irqrestore(flags);
>  
>  	if (lock_failed)
> @@ -341,7 +341,7 @@ static void __up_console_sem(unsigned long ip)
>  	mutex_release(&console_lock_dep_map, ip);
>  
>  	printk_safe_enter_irqsave(flags);
> -	up(&console_sem);
> +	mutex_unlock(&console_sem);

mutex_unlock() does not like when its called from IRQ, so this is not
going to work very well.