On Fri, Jul 23, 2021 at 08:52:50AM +0200, Manfred Spraul wrote:
> Hi Alan,
Hi.
> On 7/23/21 4:08 AM, 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.
>
> 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).
> > > + 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.
>
> 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.
> > > + }
> > > + }
> > > +
> > > + 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().
>
> 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.
Alan Stern
