Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads
From: Manfred Spraul
Date: Fri Jul 23 2021 - 02:52:57 EST
Hi Alan,
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.
+ 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)
+ }
+ }
+
+ 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.
--
Manfred