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From:   John Ogness <john.ogness@linutronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
In-Reply-To: <X8/jS9k/eMIL+Acw@jagdpanzerIV.localdomain>
References: <20201201205341.3871-1-john.ogness@linutronix.de> <20201201205341.3871-3-john.ogness@linutronix.de> <X8n9a2DWUFE/giyB@alley> <X8/jS9k/eMIL+Acw@jagdpanzerIV.localdomain>
Date:   Tue, 08 Dec 2020 23:36:44 +0106
Message-ID: <875z5c9bhn.fsf@jogness.linutronix.de>
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On 2020-12-09, Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:
>> Sigh, atomic64_read() uses a spin lock in the generic implementation
>> that is used on some architectures.
>
> Oh... So on those archs prb is not lockless in fact, it actually
> takes the spin_lock each time we read the descriptor state?
>
> 	desc_read()
> 	  atomic_long_read(state_var)
> 	    atomic64_read()
> 	      raw_spin_lock_irqsave(lock, flags)
> 	        << NMI panic >>
>
> Am I missing something?

For the state variable we chose atomic_long_t instead of atomic64_t for
this reason. atomic_long_t operations are available atomically on all
architectures. However, for clear_seq we need 64-bit (even on 32-bit
machines). The seqcount_latch is an excellent solution here since
clear_seq does not require lockless writers.

John Ogness