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

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Date:   Wed, 9 Dec 2020 10:04:03 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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 (20/12/08 23:36), John Ogness wrote:
> 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.

Right. Looking more at Kconfigs, it seems that when atomic_long_t is
atomic64 (64BIT) then GENERIC_ATOMIC64 is not selected. Those archs
that select GENERIC_ATOMIC64 unconditionally all seem to be 32-bit.

Thanks.

	-ss