On Thu, Jul 22, 2021 at 10:08:46PM -0400, Alan Stern wrote:
> 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.
Good point. The fact that wwe hold global_lock means that global_flag
cannot be set, which means that we can unconditionally acquire the
per-foo lock and release global_lock.
> > + spin_lock(&fp->f_lock);
> > + spin_unlock(&global_lock);
> > + gf = false;
> > + }
> > + do_something(fp);
> > + if (fg)
>
> Should be gf, not fg.
And we can also eliminate gf and its typo.
> > + 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.
But also with the smp_load_acquire() of global_flag, right?
> > + 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.
As suggested later in this thread, I have added a comment.
> > + }
> > + }
> > +
> > + 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?
I have removed this comment. The last shred of reason for it went away
with the gf local variable.
> > + 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().
I was thinking along those lines, but I clearly wasn't thinking very
clearly. :-/
> > +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?
Good catch, and I took care of this by rewriting this paragraph.
Likely introducing other typos in the process, but so it goes.
> > +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().
Agreed.
Thanx, Paul
