linux-kernel - Re: [RFC] seqlock,lockdep: Add lock primitives to read

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Date:	Wed, 30 Mar 2011 21:17:29 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	peterz@...radead.org
Cc:	rostedt@...dmis.org, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] seqlock,lockdep: Add lock primitives to read_seqbegin().

Peter Zijlstra wrote:
> >  Also, I assume you meant to call
> > spin_acquire() before entering the spin state (as with
> > 
> >   static inline void __raw_spin_lock(raw_spinlock_t *lock)
> >   {
> >         preempt_disable();
> >         spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
> >         LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
> >   }
> > 
> > . Otherwise, lockdep cannot report it when hit this bug upon the first call to
> > this function). 
> 
> Huh no, of course not, a seqlock read side cannot contend in the classic
> sense.

I couldn't understand what 'contend' means. I think

  static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  {
  	unsigned ret;
  repeat:
  	ret = sl->sequence;
  	smp_rmb();
  	if (unlikely(ret & 1)) {
  		cpu_relax();
  		goto repeat;
  	}
  	return ret;
  }

is equivalent (except that above one will not write to any kernel memory) to

  static __always_inline unsigned read_seqbegin(seqlock_t *sl)
  {
  	unsigned ret;
  	unsigned long flags;
  	spin_lock_irqsave(&sl->lock, flags);
  	ret = sl->sequence;
  	spin_unlock_irqrestore(&sl->lock, flags);
  	return ret;
  }

because read_seqbegin() cannot return to the reader until the writer (if there
is one) calls write_sequnlock().

  static inline void write_seqlock(seqlock_t *sl)
  {
  	spin_lock(&sl->lock);
  	++sl->sequence;
  	smp_wmb();
  }
  static inline void write_sequnlock(seqlock_t *sl)
  {
  	smp_wmb();
  	sl->sequence++;
  	spin_unlock(&sl->lock);
  }

Don't we call this situation (a reader thread temporarily behaves like a writer
thread who writes nothing) as 'contended'?

Anyway, could you show me read_seqbegin2()/read_seqretry2() for testing with
locktest module?

Regards.
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