Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Jul 23, 2021 at 01:08:20PM -0400, Alan Stern wrote:
> On Fri, Jul 23, 2021 at 09:30:08AM -0700, Paul E. McKenney wrote:
> > How about like this?
> > 
> > 							Thanx, Paul
> 
> Generally a lot better, but still at least one issue.
> 
> > ------------------------------------------------------------------------
> > 
> > Lock-Protected Writes With Heuristic Lockless Reads
> > ---------------------------------------------------
> > 
> > For another example, suppose that the code can normally make use of
> > a per-data-structure lock, but there are times when a global lock
> > is required.  These times are indicated via a global flag.  The code
> > might look as follows, and is based loosely on nf_conntrack_lock(),
> > nf_conntrack_all_lock(), and nf_conntrack_all_unlock():
> > 
> > 	bool global_flag;
> > 	DEFINE_SPINLOCK(global_lock);
> > 	struct foo {
> > 		spinlock_t f_lock;
> > 		int f_data;
> > 	};
> > 
> > 	/* All foo structures are in the following array. */
> > 	int nfoo;
> > 	struct foo *foo_array;
> > 
> > 	void do_something_locked(struct foo *fp)
> > 	{
> > 		/* IMPORTANT: Heuristic plus spin_lock()! */
> > 		if (!data_race(global_flag)) {
> > 			spin_lock(&fp->f_lock);
> > 			if (!smp_load_acquire(&global_flag)) {
> > 				do_something(fp);
> > 				spin_unlock(&fp->f_lock);
> > 				return;
> > 			}
> > 			spin_unlock(&fp->f_lock);
> > 		}
> > 		spin_lock(&global_lock);
> > 		/* global_lock held, thus global flag cannot be set. */
> > 		spin_lock(&fp->f_lock);
> > 		spin_unlock(&global_lock);
> > 		/*
> > 		 * global_flag might be set here, but begin_global()
> > 		 * will wait for ->f_lock to be released.
> > 		 */
> > 		do_something(fp);
> > 		spin_lock(&fp->f_lock);
> 
> spin_unlock.

Good eyes, fixed.

> > }
> > 
> > 	void begin_global(void)
> > 	{
> > 		int i;
> > 
> > 		spin_lock(&global_lock);
> > 		WRITE_ONCE(global_flag, true);
> > 		for (i = 0; i < nfoo; i++) {
> > 			/*
> > 			 * Wait for pre-existing local locks.  One at
> > 			 * a time to avoid lockdep limitations.
> > 			 */
> > 			spin_lock(&fp->f_lock);
> > 			spin_unlock(&fp->f_lock);
> > 		}
> > 	}
> > 
> > 	void end_global(void)
> > 	{
> > 		smp_store_release(&global_flag, false);
> > 		spin_unlock(&global_lock);
> > 	}
> > 
> > All code paths leading from the do_something_locked() function's first
> > read from global_flag acquire a lock, so endless load fusing cannot
> > happen.
> > 
> > If the value read from global_flag is true, then global_flag is
> > rechecked while holding ->f_lock, which, if global_flag is now false,
> > prevents begin_global() from completing.  It is therefore safe to invoke
> > do_something().
> > 
> > Otherwise, if either value read from global_flag is true, then after
> > global_lock is acquired global_flag must be false.  The acquisition of
> > ->f_lock will prevent any call to begin_global() from returning, which
> > means that it is safe to release global_lock and invoke do_something().
> > 
> > For this to work, only those foo structures in foo_array[] may be passed
> > to do_something_locked().  The reason for this is that the synchronization
> > with begin_global() relies on momentarily holding the lock of each and
> > every foo structure.
> 
> This doesn't mention the reason for the acquire-release
> synchronization of global_flag.  It's needed because work done between
> begin_global() and end_global() can affect a foo structure without
> holding its private f_lock member, and we want all such work to be
> visible to other threads when they call do_something_locked() later.

Like this added paragraph at the end?

	The smp_load_acquire() and smp_store_release() are required
	because changes to a foo structure between calls to begin_global()
	and end_global() are carried out without holding that structure's
	->f_lock.  The smp_load_acquire() and smp_store_release()
	ensure that the next invocation of do_something() from the call
	to do_something_locked() that acquires that ->f_lock will see
	those changes.

							Thanx, Paul