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jump_label: Fix static_key_slow_dec() yet again
While commit83ab38ef0a
("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 to83ab38ef0a
. Add a few WARNs for the paranoid. Fixes:83ab38ef0a
("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>
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@ -168,7 +168,7 @@ bool static_key_slow_inc_cpuslocked(struct static_key *key)
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jump_label_update(key);
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/*
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* Ensure that when static_key_fast_inc_not_disabled() or
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* static_key_slow_try_dec() observe the positive value,
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* static_key_dec_not_one() observe the positive value,
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* they must also observe all the text changes.
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*/
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atomic_set_release(&key->enabled, 1);
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@ -250,7 +250,7 @@ void static_key_disable(struct static_key *key)
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}
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EXPORT_SYMBOL_GPL(static_key_disable);
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static bool static_key_slow_try_dec(struct static_key *key)
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static bool static_key_dec_not_one(struct static_key *key)
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{
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int v;
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@ -274,6 +274,14 @@ static bool static_key_slow_try_dec(struct static_key *key)
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* enabled. This suggests an ordering problem on the user side.
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*/
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WARN_ON_ONCE(v < 0);
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/*
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* Warn about underflow, and lie about success in an attempt to
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* not make things worse.
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*/
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if (WARN_ON_ONCE(v == 0))
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return true;
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if (v <= 1)
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return false;
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} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
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@ -284,15 +292,27 @@ static bool static_key_slow_try_dec(struct static_key *key)
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static void __static_key_slow_dec_cpuslocked(struct static_key *key)
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{
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lockdep_assert_cpus_held();
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int val;
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if (static_key_slow_try_dec(key))
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if (static_key_dec_not_one(key))
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return;
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guard(mutex)(&jump_label_mutex);
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if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
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val = atomic_read(&key->enabled);
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/*
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* It should be impossible to observe -1 with jump_label_mutex held,
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* see static_key_slow_inc_cpuslocked().
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*/
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if (WARN_ON_ONCE(val == -1))
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return;
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/*
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* Cannot already be 0, something went sideways.
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*/
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if (WARN_ON_ONCE(val == 0))
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return;
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if (atomic_dec_and_test(&key->enabled))
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jump_label_update(key);
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else
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WARN_ON_ONCE(!static_key_slow_try_dec(key));
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}
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static void __static_key_slow_dec(struct static_key *key)
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@ -329,7 +349,7 @@ void __static_key_slow_dec_deferred(struct static_key *key,
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{
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STATIC_KEY_CHECK_USE(key);
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if (static_key_slow_try_dec(key))
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if (static_key_dec_not_one(key))
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return;
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schedule_delayed_work(work, timeout);
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