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jump_label: Fix static_key_slow_dec() yet again

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While commit 83ab38ef0a ("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
83ab38ef0a. 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>
This commit is contained in:
Peter Zijlstra 2024-09-09 12:50:09 +02:00
parent fe513c2ef0
commit 1d7f856c2c
1 changed files with 27 additions and 7 deletions
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34
kernel/jump_label.c
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@ -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);