linux-kernel - Re: Process Hang in __read_seqcount

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Message-ID: <CAN8Q1EeqmHMHD-VUgr7Wzv0Z_DAD1SrEniftg==L14d+hGwu+Q@mail.gmail.com>
Date:	Mon, 22 Oct 2012 10:25:42 -0700
From:	Peter LaDow <petela@...ougs.wsu.edu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Process Hang in __read_seqcount_begin

On Mon, Oct 22, 2012 at 10:01 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> This looks like a corruption of s->sequence, and is value is odd, even
> if no writer is alive.
>
> Does local_bh_disable() disables preemption on RT ?

Hmmm....

With PREEMPT_RT_FULL defined (as we have):

void local_bh_disable(void)
{
        migrate_disable();
        current->softirq_nestcnt++;
}

And the RT patches add the following:

#ifdef CONFIG_PREEMPT_RT_FULL
# define preempt_disable_rt()          preempt_disable()
# define preempt_enable_rt()           preempt_enable()
# define preempt_disable_nort()                do { } while (0)
# define preempt_enable_nort()         do { } while (0)
# ifdef CONFIG_SMP
   extern void migrate_disable(void);
   extern void migrate_enable(void);
# else /* CONFIG_SMP */
#  define migrate_disable()            do { } while (0)
#  define migrate_enable()             do { } while (0)
# endif /* CONFIG_SMP */
#else
# define preempt_disable_rt()          do { } while (0)
# define preempt_enable_rt()           do { } while (0)
# define preempt_disable_nort()                preempt_disable()
# define preempt_enable_nort()         preempt_enable()
# define migrate_disable()             preempt_disable()
# define migrate_enable()              preempt_enable()
#endif

And since we are not SMP, local_bh_disable() essentially does nothing.
 These definitions are consistent across all the RT patches, up to
3.6.2-rt4 (as far as I can tell).

Now, is preemption required to be disabled in non-SMP systems?
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