linux-kernel - Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64

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Date:   Tue, 08 Dec 2020 23:36:44 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t

On 2020-12-09, Sergey Senozhatsky <sergey.senozhatsky@...il.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