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Message-ID: <20210723020846.GA26397@rowland.harvard.edu>
Date: Thu, 22 Jul 2021 22:08:46 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, parri.andrea@...il.com,
will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
luc.maranget@...ia.fr, akiyks@...il.com,
Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for
heuristic lockless reads
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@...orfullife.com>
> [ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
> Signed-off-by: Paul E. McKenney <paulmck@...nel.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.
> + 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
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