How to do proper locking

[Hans Petter Selasky]

On Thursday 04 August 2005 20:15, John Baldwin wrote:
> On Thursday 04 August 2005 12:50 pm, Hans Petter Selasky wrote:
> > On Thursday 04 August 2005 15:53, John Baldwin wrote:
> > > On Thursday 04 August 2005 07:40 am, Hans Petter Selasky wrote:
> > > > On Wednesday 03 August 2005 19:21, John Baldwin wrote:
> > > > > On Tuesday 02 August 2005 06:23 pm, Hans Petter Selasky wrote:
> > > > > > Hi,
> > > > > >
> > > > > > I am looking for a safe way to access structures that can be
> > > > > > freed. The solution I am looking for must not:
> > > > > >
> > > > > > - hinder scaleability
> > > > > > - lead to use of a single lock
> > > > > > - lead to lock order reversal
> > > >
> > > > There is one more thing. When the structure is freed it should not
> > > > block waiting for any reference counts.
> > >
> > > crhold() is always called when the caller knows that it has a reference
> > > to cr that can't go away.  Thus, in the case of p_ucred, one holds the
> > > associated PROC_LOCK() while doing crhold().  After the crhold() you
> > > can drop the proc lock, so that lock is only held for a very short
> > > period of time.  You then have your own reference to the cred structure
> > > that is valid until you call crfree().  In the case of
> > > curthread->td_ucred, the reference is valid until curthread releases it
> > > on a cred update at the start of a syscall, so your reference is valid
> > > without needing locks for the life of a syscall.  The key here is that
> > > you solve this problem at a higher level, not within the structure
> > > itself.
> >
> > Yes, this works with mbufs, crhold() and alike, where one doesn't
> > actually free the object until the refcount reaches zero. But I am
> > speaking about destroying an object while the refcount is non-zero. Do
> > you see the difference?
> >
> > There are two ways to handle this:
> >
> > 1) blocking: wait until the refcount reaches zero.
> >
> > 2) nonblocking: increment some other refcount that the
> >    callback checks before accessing any data.
> >
> >
> > Do you agree that method 2 is going to:
> >
> > - avoid deadlock
> > - avoid use of sx-locks, locks that allow calls to msleep(),
> >   to keep synchronization while destroying objects
> >   with refcounts.
>
> Not always. :)
>
> > When calling functions like "callout_stop()", it is likely that one is
> > holding a lock to prevent other code from calling "callout_start()" on
> > the same callback before "callout_stop()" has been called. Now, if
> > "callout_stop()" sleeps, the lock that is held while calling this
> > function, must allow calls msleep(). So this must be an sx-lock (see "man
> > sx"). Now if one routine sleeps, then any callers of this routine must
> > sleep too. So this affects the synchronization of the whole system.
> >
> > Isn't nonblocking operation preferred over blocking operation?
>
> You can call callout_stop() while holding the lock, then do a
> callout_drain() later when you are prepared to block.  This is what I'm
> doing with ethernet drivers right now that typically call callout_stop in
> their foo_stop() routine so detach looks like:
>
>  FOO_LOCK(sc);
>  foo_stop(sc); // calls callout_stop()
>  FOO_UNLOCK(sc);
>  callout_drain(&sc->foo_stat_ch);
>

This is only going to work if you are detaching. But consider the following:

  FOO_LOCK(sc);
  callout_stop();
  callout_start();
  FOO_UNLOCK(sc);

Your solution is only spinning halfway around. What I say is that you need 
some more parameters passed to callbacks, so that one can check if it has 
been cancelled before proceeding.

>
> > No, the callback functions read/write/ioctl/poll can be called after that
> > "destroy_dev()" has been called.
>
> Nope.  Every call to read/write/ioctl bumps a reference count on the
> underlying dev_t and destroy_dev() blocks until the reference count drops
> to zero.

Yes, I see that "dev_refthread()" and "dev_relthread()" are called from 
"devfs_xxx()", but "destroy_dev()" will not wait for these refcounts to reach 
zero unless the "d_purge" field is initialized in "struct cdevsw". And I 
cannot see that "prep_cdevs()" is setting any default value for this field. 
So by default "destroy_dev()" is non-blocking which is right.

I am a bit lazy today, so I did:

cat `find /usr/src/sys/dev/*` | grep d_purge

And got two hits. This feature is not used at all.

>
> >
> > What I want is that the kernel provides some routine that can do locking
> > based on a structure like below. Also the kernel must provide some
> > routines to allocate, free and read a refcount.
> >
> > struct callback_args {
> >   void *sc;
> >   struct mtx *p_mtx;
> >   u_int32_t refcount_copy;
> >   u_int32_t *p_refcount;
> > };
> >
> > Then I want routines like "callout_reset()", "make_dev()",
> > "bus_setup_intr()" and so on, to pass these four parameters to the
> > callback function, either directly, or like a pointer to this structure.
>
> You are just going to move the blocking behavior into contention on your
> locks, you aren't going to actually remove it anywhere

Yes, that is correct, but from what I understand it is inherently more 
efficient to wait for a mutex of type "struct mtx", than it is to use 
msleep/wakeup, which is the current way of doing it, according to "man sx".

> , and since all users 
> of the structures have to do more checking at runtime

I don't think so.

> , I think you will end up increasing overhead.

Let's consider a real example, for example "devfs_read_f()":

devfs_read_f(...)
{

        dev_refthread(dev);

        error = dsw->d_read(dev, uio, ioflag);

        dev_relthread(dev);

        return (error);
}

Per "devfs_read_f()" one has got to lock/unlock two times. It involves two 
refcount reads and two refcount writes, which means that the CPU cache will 
be invalidated.


Now consider my proposal. There is no more need for 
dev_refthread/dev_relthread. Instead the callback, "d_read()" will do the 
checking. This consist of:

d_read()
{
    mtx_lock(args->mtx);

    if(args->refcount_copy == atomic_read(args->refcount_p))
    {
      /* valid */
    }

    mtx_unlock(args->mtx);

    return;
}

Per "devfs_read_f()" one has got to lock/unlock one time. It involves two 
refcount reads and no refcount writes.

Well, isn't this 200 percent faster?


The only overhead is that the "args" structure must be copied into the 
file-descriptor of the current thread while holding some global lock. But 
once that is done, there is no need to lock this global lock again. So if one 
reads from a device more times than one opens it, it is going to pay off.

To cut down on the number of locks in the kernel, external routines accessing 
"dev" should get a copy of "args" while holding a global lock. Then use the 
"args" to lock "dev". If it fails, due to some paralell thread calling 
"dev_destroy()", just repeat.

So this "args" structure I suggest, is like a ticket to get valid access to 
some callback, and should be copied to the callers that need this access.

The following looks like a nice way to implement this:

if(enter_callback(args))
{
   /* valid */
   exit_callback(args);

   /* free to do other things */

   if(enter_callback(args))
   {
        /* still valid - only one check required */
        exit_callback(args);
   }
}

So what do you think ?

--HPS