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

[Alan Stern]

On Wed, Jul 21, 2021 at 02:10:01PM -0700, Paul E. McKenney wrote:
> This commit adds example code for heuristic lockless reads, based loosely
> on the sem_lock() and sem_unlock() functions.
> 
> Reported-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> [ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  .../Documentation/access-marking.txt          | 94 +++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/tools/memory-model/Documentation/access-marking.txt b/tools/memory-model/Documentation/access-marking.txt
> index 58bff26198767..be7d507997cf8 100644
> --- a/tools/memory-model/Documentation/access-marking.txt
> +++ b/tools/memory-model/Documentation/access-marking.txt
> @@ -319,6 +319,100 @@ of the ASSERT_EXCLUSIVE_WRITER() is to allow KCSAN to check for a buggy
>  concurrent lockless write.
>  
>  
> +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)
> +	{
> +		bool gf = true;
> +
> +		/* 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);
> +		/* Lock held, thus global flag cannot change. */
> +		if (!global_flag) {

How can global_flag ever be true at this point?  The only line of code 
that sets it is in begin_global() below, it only runs while global_lock 
is held, and global_flag is set back to false before the lock is 
released.

> +			spin_lock(&fp->f_lock);
> +			spin_unlock(&global_lock);
> +			gf = false;
> +		}
> +		do_something(fp);
> +		if (fg)

Should be gf, not fg.

> +			spin_unlock(&global_lock);
> +		else
> +			spin_lock(&fp->f_lock);
> +	}
> +
> +	void begin_global(void)
> +	{
> +		int i;
> +
> +		spin_lock(&global_lock);
> +		WRITE_ONCE(global_flag, true);

Why does this need to be WRITE_ONCE?  It still races with the first read 
of global_flag above.

> +		for (i = 0; i < nfoo; i++) {
> +			/* Wait for pre-existing local locks. */
> +			spin_lock(&fp->f_lock);
> +			spin_unlock(&fp->f_lock);

Why not acquire all the locks here and release all of them in 
end_global()?  Then global_flag wouldn't need acquire-release 
sychronization.

> +		}
> +	}
> +
> +	void end_global(void)
> +	{
> +		smp_store_release(&global_flag, false);
> +		/* Pre-existing global lock acquisitions will recheck. */

What does that comment mean?  How can there be any pre-existing global 
lock acquisitions when we hold the lock right now?

> +		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 global_lock, which prevents global_flag from changing.
> +If this recheck finds that global_flag is now false, the acquisition

Again, how can't global_flag be false now?

Did you originally have in mind some sort of scheme in which 
begin_global() would release global_lock before returning and 
end_global() would acquire global_lock before clearing global_flag?  But 
I don't see how that could work without changes to do_something_locked().

> +of ->f_lock prior to the release of global_lock will result in any subsequent
> +begin_global() invocation waiting to acquire ->f_lock.
> +
> +On the other hand, if the value read from global_flag is false, then
> +global_flag, then rechecking under ->f_lock combined with synchronization
---^^^^^^^^^^^^^^^^^^

Typo?

> +with begin_global() guarantees than any erroneous read will cause the
> +do_something_locked() function's first do_something() invocation to happen
> +before begin_global() returns.  The combination of the smp_load_acquire()
> +in do_something_locked() and the smp_store_release() in end_global()
> +guarantees that either the do_something_locked() function's first
> +do_something() invocation happens after the call to end_global() or that
> +do_something_locked() acquires global_lock() and rechecks under the lock.

This last sentence also makes no sense unless you imagine dropping 
global_lock between begin_global() and end_global().

Alan