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

On Fri, Jul 23, 2021 at 09:05:54AM -0400, Alan Stern wrote:

On Fri, Jul 23, 2021 at 08:52:50AM +0200, Manfred Spraul wrote:

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Hi Alan,

Hi.

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On 7/23/21 4:08 AM, Alan Stern wrote:

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On Wed, Jul 21, 2021 at 02:10:01PM -0700, Paul E. McKenney wrote:

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This commit adds example code for heuristic lockless reads, based loosely
on the sem_lock() and sem_unlock() functions.

Reported-by: Manfred Spraul <redacted>
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
  .../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.

It can't be true. The code is a simplified version of the algorithm in
ipc/sem.c.

For the ipc/sem.c, global_flag can remain true even after dropping
global_lock.

When transferring the approach to nf_conntrack_core, I didn't notice that
nf_conntrack doesn't need a persistent global_flag.

Thus the recheck after spin_lock(&global_lock) is not needed.

In fact, since global_flag is true if and only if global_lock is locked, 
perhaps it can be removed entirely and replaced with 
spin_is_locked(&global_lock).

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

Should be gf, not fg.

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+			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.

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+		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.

From my understanding:
spin_lock contains preempt_count_add, thus you can't acquire more than 255
spinlocks (actually 245, the warning limit is 10 below 255)

It might be worth mentioning this in a code comment.  Or in the 
accompanying text.

As noted earlier, done.

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+		}
+	}
+
+	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?

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+		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().

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+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?

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+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().

ipc/sem.c does that and needs that, nf_conntrack doesn't use this.

Given all these issues, it seems like this patch needs to be re-written.

How about like this?

							Thanx, Paul

------------------------------------------------------------------------

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);
}

	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.