jump_label: Fix static_key_slow_dec() yet again

While commit 83ab38ef0a0b ("jump_label: Fix concurrency issues in
static_key_slow_dec()") fixed one problem, it created yet another,
notably the following is now possible:

  slow_dec
    if (try_dec) // dec_not_one-ish, false
    // enabled == 1
                                slow_inc
                                  if (inc_not_disabled) // inc_not_zero-ish
                                  // enabled == 2
                                    return

    guard((mutex)(&jump_label_mutex);
    if (atomic_cmpxchg(1,0)==1) // false, we're 2

                                slow_dec
                                  if (try-dec) // dec_not_one, true
                                  // enabled == 1
                                    return
    else
      try_dec() // dec_not_one, false
      WARN

Use dec_and_test instead of cmpxchg(), like it was prior to
83ab38ef0a0b. Add a few WARNs for the paranoid.

Fixes: 83ab38ef0a0b ("jump_label: Fix concurrency issues in static_key_slow_dec()")
Reported-by: "Darrick J. Wong" <djwong@kernel.org>
Tested-by: Klara Modin <klarasmodin@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 6dc76b590703edd263f8e88c9f15da09f4b66aa7..93a822d3c468ca2de59eb5a24cd227c9f7083fa4 100644 (file)
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -168,7 +168,7 @@ bool static_key_slow_inc_cpuslocked(struct static_key *key)
                jump_label_update(key);
                /*
                 * Ensure that when static_key_fast_inc_not_disabled() or
-                * static_key_slow_try_dec() observe the positive value,
+                * static_key_dec_not_one() observe the positive value,
                 * they must also observe all the text changes.
                 */
                atomic_set_release(&key->enabled, 1);
@@ -250,7 +250,7 @@ void static_key_disable(struct static_key *key)
 }
 EXPORT_SYMBOL_GPL(static_key_disable);
 
-static bool static_key_slow_try_dec(struct static_key *key)
+static bool static_key_dec_not_one(struct static_key *key)
 {
        int v;
 
@@ -274,6 +274,14 @@ static bool static_key_slow_try_dec(struct static_key *key)
                 * enabled. This suggests an ordering problem on the user side.
                 */
                WARN_ON_ONCE(v < 0);
+
+               /*
+                * Warn about underflow, and lie about success in an attempt to
+                * not make things worse.
+                */
+               if (WARN_ON_ONCE(v == 0))
+                       return true;
+
                if (v <= 1)
                        return false;
        } while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
@@ -284,15 +292,27 @@ static bool static_key_slow_try_dec(struct static_key *key)
 static void __static_key_slow_dec_cpuslocked(struct static_key *key)
 {
        lockdep_assert_cpus_held();
+       int val;
 
-       if (static_key_slow_try_dec(key))
+       if (static_key_dec_not_one(key))
                return;
 
        guard(mutex)(&jump_label_mutex);
-       if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
+       val = atomic_read(&key->enabled);
+       /*
+        * It should be impossible to observe -1 with jump_label_mutex held,
+        * see static_key_slow_inc_cpuslocked().
+        */
+       if (WARN_ON_ONCE(val == -1))
+               return;
+       /*
+        * Cannot already be 0, something went sideways.
+        */
+       if (WARN_ON_ONCE(val == 0))
+               return;
+
+       if (atomic_dec_and_test(&key->enabled))
                jump_label_update(key);
-       else
-               WARN_ON_ONCE(!static_key_slow_try_dec(key));
 }
 
 static void __static_key_slow_dec(struct static_key *key)
@@ -329,7 +349,7 @@ void __static_key_slow_dec_deferred(struct static_key *key,
 {
        STATIC_KEY_CHECK_USE(key);
 
-       if (static_key_slow_try_dec(key))
+       if (static_key_dec_not_one(key))
                return;
 
        schedule_delayed_work(work, timeout);