Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Feb 12, 2009 at 02:38:26PM -0500, Mathieu Desnoyers wrote:
> > Replying to a separate portion of the mail with less CC :
> > 
> > 
> > > On Thu, Feb 12, 2009 at 02:05:39AM -0500, Mathieu Desnoyers wrote:
> > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > > On Wed, Feb 11, 2009 at 11:08:24PM -0500, Mathieu Desnoyers wrote:
> > > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > > > > On Wed, Feb 11, 2009 at 04:35:49PM -0800, Paul E. McKenney wrote:
> > > > > > > > On Wed, Feb 11, 2009 at 04:42:58PM -0500, Mathieu Desnoyers wrote:
> > > > > > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > > 
> > > > > [ . . . ]
> > > > > 
> > > > > > > And I had bugs in my model that allowed the rcu_read_lock() model
> > > > > > > to nest indefinitely, which overflowed into the top bit, messing
> > > > > > > things up.  :-/
> > > > > > > 
> > > > > > > Attached is a fixed model.  This model validates correctly (woo-hoo!).
> > > > > > > Even better, gives the expected error if you comment out line 180 and
> > > > > > > uncomment line 213, this latter corresponding to the error case I called
> > > > > > > out a few days ago.
> > > > > > > 
> > > > > > 
> > > > > > Great ! :) I added this version to the git repository, hopefully it's ok
> > > > > > with you ?
> > > > > 
> > > > > Works for me!
> > > > > 
> > > > > > > I will play with removing models of mb...
> > > > > > 
> > > > > > OK, I see you already did..
> > > > > 
> > > > > I continued this, and surprisingly few are actually required, though
> > > > > I don't fully trust the modeling of removed memory barriers.
> > > > 
> > > > On my side I cleaned up the code a lot, and actually added some barriers
> > > > ;) Especially in the busy loops, where we expect the other thread's
> > > > value to change eventually between iterations. A smp_rmb() seems more
> > > > appropriate that barrier(). I also added a lot of comments about
> > > > barriers in the code, and made the reader side much easier to review.
> > > > 
> > > > Please feel free to comment on my added code comments.
> > > 
> > > The torture test now looks much more familiar.  ;-)
> > > 
> > > I fixed some compiler warnings (in my original, sad to say), added an
> > > ACCESS_ONCE() to rcu_read_lock() (also in my original),
> > 
> > Yes, I thought about this ACCESS_ONCE during my sleep.. just did not
> > have to to update the source yet. :)
> > 
> > Merged. Thanks !
> > 
> > [...]
> > 
> > > --- a/urcu.c
> > > +++ b/urcu.c
> > > @@ -99,7 +99,8 @@ static void force_mb_single_thread(pthread_t tid)
> > >  	 * BUSY-LOOP.
> > >  	 */
> > >  	while (sig_done < 1)
> > > -		smp_rmb();	/* ensure we re-read sig-done */
> > > +		barrier();	/* ensure compiler re-reads sig-done */
> > > +				/* cache coherence guarantees CPU re-read. */
> > 
> > That could be a smp_rmc() ? (see other mail)
> 
> I prefer making ACCESS_ONCE() actually having the full semantics implied
> by its name.  ;-)
> 
> See patch at end of this email.
> 

See my email about LOAD_REMOTE/STORE_REMOTE :)

> > >  	smp_mb();	/* read sig_done before ending the barrier */
> > >  }
> > >  
> > > @@ -113,7 +114,8 @@ static void force_mb_all_threads(void)
> > >  	if (!reader_data)
> > >  		return;
> > >  	sig_done = 0;
> > > -	smp_mb();	/* write sig_done before sending the signals */
> > > +	/* smp_mb();	write sig_done before sending the signals */
> > > +			/* redundant with barriers in pthread_kill(). */
> > 
> > Absolutely not. pthread_kill does not send a signal to self in every
> > case because the writer thread has not requirement to register itself.
> > It *could* be registered as a reader too, but does not have to.
> 
> No, not the barrier in the signal handler, but rather the barriers in
> the system call invoked by pthread_kill().
> 

The barrier implied by going through a system call does not imply cache
flushing AFAIK. So we would have to at least leave a big comment here
saying that the kernel has to provide such guarantee. So under that
comment I would leave a smp_mc();.

> > >  	for (index = reader_data; index < reader_data + num_readers; index++)
> > >  		pthread_kill(index->tid, SIGURCU);
> > >  	/*
> > > @@ -121,7 +123,8 @@ static void force_mb_all_threads(void)
> > >  	 * BUSY-LOOP.
> > >  	 */
> > >  	while (sig_done < num_readers)
> > > -		smp_rmb();	/* ensure we re-read sig-done */
> > > +		barrier();	/* ensure compiler re-reads sig-done */
> > > +				/* cache coherence guarantees CPU re-read. */
> > 
> > That could be a smp_rmc() ?
> 
> Again, prefer:
> 
> 	while (ACCESS_ONCE() < num_readers)
> 
> after upgrading ACCESS_ONCE() to provide the full semantics.
> 
> I will send a patch.
> 

I'll use a variation :

        while (LOAD_REMOTE(sig_done) < num_readers)
                cpu_relax();


> > >  	smp_mb();	/* read sig_done before ending the barrier */
> > >  }
> > >  #endif
> > > @@ -181,7 +184,8 @@ void synchronize_rcu(void)
> > >  	 * the writer waiting forever while new readers are always accessing
> > >  	 * data (no progress).
> > >  	 */
> > > -	smp_mb();
> > > +	/* smp_mb(); Don't need this one for CPU, only compiler. */
> > > +	barrier();
> > 
> > smp_mc() ?
> 
> ACCESS_ONCE().
> 

Ah, this is what I dislike about using :

  STORE_REMOTE(x, v);
...
  if (LOAD_REMOTE(y) ...)
rather than
  x = v;
  smp_mc();
  if (y ...)

We will end up in a situation where we do 2 cache flushes rather than a
single one. So wherever possible, I would be tempted to leave the
smp_mc().


> > >  
> > >  	switch_next_urcu_qparity();	/* 1 -> 0 */
> > >  
> > 
> > Side-note :
> > on archs without cache coherency, all smp_[rw ]mb would turn into a
> > cache flush.
> 
> So I might need more in my ACCESS_ONCE() below.
> 
> Add .gitignore files, and redefine accesses in terms of a new
> ACCESS_ONCE().
> 

I'll merge the .gitignore file, thanks,

Please see my updated git tree.

Mathieu

> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  .gitignore              |    9 +++++++++
>  formal-model/.gitignore |    3 +++
>  urcu.c                  |   10 ++++------
>  urcu.h                  |   12 ++++++++++++
>  4 files changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/.gitignore b/.gitignore
> new file mode 100644
> index 0000000..29aa7e5
> --- /dev/null
> +++ b/.gitignore
> @@ -0,0 +1,9 @@
> +test_rwlock_timing
> +test_urcu
> +test_urcu_timing
> +test_urcu_yield
> +urcu-asm.o
> +urcu.o
> +urcutorture
> +urcutorture-yield
> +urcu-yield.o
> diff --git a/formal-model/.gitignore b/formal-model/.gitignore
> new file mode 100644
> index 0000000..49fdd8a
> --- /dev/null
> +++ b/formal-model/.gitignore
> @@ -0,0 +1,3 @@
> +pan
> +pan.*
> +urcu.spin.trail
> diff --git a/urcu.c b/urcu.c
> index a696439..f61d4c3 100644
> --- a/urcu.c
> +++ b/urcu.c
> @@ -98,9 +98,8 @@ static void force_mb_single_thread(pthread_t tid)
>  	 * Wait for sighandler (and thus mb()) to execute on every thread.
>  	 * BUSY-LOOP.
>  	 */
> -	while (sig_done < 1)
> -		barrier();	/* ensure compiler re-reads sig-done */
> -				/* cache coherence guarantees CPU re-read. */
> +	while (ACCESS_ONCE(sig_done) < 1)
> +		continue;
>  	smp_mb();	/* read sig_done before ending the barrier */
>  }
>  
> @@ -122,9 +121,8 @@ static void force_mb_all_threads(void)
>  	 * Wait for sighandler (and thus mb()) to execute on every thread.
>  	 * BUSY-LOOP.
>  	 */
> -	while (sig_done < num_readers)
> -		barrier();	/* ensure compiler re-reads sig-done */
> -				/* cache coherence guarantees CPU re-read. */
> +	while (ACCESS_ONCE(sig_done) < num_readers)
> +		continue;
>  	smp_mb();	/* read sig_done before ending the barrier */
>  }
>  #endif
> diff --git a/urcu.h b/urcu.h
> index 79d9464..dd040a5 100644
> --- a/urcu.h
> +++ b/urcu.h
> @@ -98,6 +98,9 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
>  /* Nop everywhere except on alpha. */
>  #define smp_read_barrier_depends()
>  
> +#define CONFIG_ARCH_CACHE_COHERENT
> +#define cpu_relax barrier
> +
>  /*
>   * Prevent the compiler from merging or refetching accesses.  The compiler
>   * is also forbidden from reordering successive instances of ACCESS_ONCE(),
> @@ -110,7 +113,16 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
>   * use is to mediate communication between process-level code and irq/NMI
>   * handlers, all running on the same CPU.
>   */
> +#ifdef CONFIG_ARCH_CACHE_COHERENT
>  #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> +#else /* #ifdef CONFIG_ARCH_CACHE_COHERENT */
> +#define ACCESS_ONCE(x)     ({ \
> +				typeof(x) _________x1; \
> +				_________x1 = (*(volatile typeof(x) *)&(x)); \
> +				cpu_relax(); \
> +				(_________x1); \
> +				})
> +#endif /* #else #ifdef CONFIG_ARCH_CACHE_COHERENT */
>  
>  /**
>   * rcu_dereference - fetch an RCU-protected pointer in an
> 

-- 
Mathieu Desnoyers
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