Messages in this thread Patch in this message | | | Date | Tue, 19 Mar 2019 13:18:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/static_key: Fix false positive warnings on concurrent dec/inc |
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On Mon, Mar 18, 2019 at 02:58:14PM -0700, Jakub Kicinski wrote: > Even though the atomic_dec_and_mutex_lock() in > __static_key_slow_dec_cpuslocked() can never see a negative > value in key->enabled the subsequent sanity check is re-reading > key->enabled, which may have been set to -1 in the meantime by > static_key_slow_inc_cpuslocked().
A little extra detail might not hurt, or a diagram or something.
> Instead of using -1 as a "enable in progress" constant use > -0xffff, this way we can still treat smaller negative values > as errors.
Those offset games always hurt my brain, but see below.
> Fixes: 4c5ea0a9cd02 ("locking/static_key: Fix concurrent static_key_slow_inc()") > Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com> > --- > kernel/jump_label.c | 21 ++++++++++----------- > 1 file changed, 10 insertions(+), 11 deletions(-) > > diff --git a/kernel/jump_label.c b/kernel/jump_label.c > index bad96b476eb6..4a227e70a8f3 100644 > --- a/kernel/jump_label.c > +++ b/kernel/jump_label.c > @@ -89,7 +89,7 @@ static void jump_label_update(struct static_key *key); > int static_key_count(struct static_key *key) > { > /* > - * -1 means the first static_key_slow_inc() is in progress. > + * -0xffff means the first static_key_slow_inc() is in progress. > * static_key_enabled() must return true, so return 1 here. > */ > int n = atomic_read(&key->enabled); > @@ -125,7 +125,10 @@ void static_key_slow_inc_cpuslocked(struct static_key *key) > > jump_label_lock(); > if (atomic_read(&key->enabled) == 0) { > - atomic_set(&key->enabled, -1); > + /* Use a large enough negative number so we can still > + * catch underflow bugs in static_key_slow_dec(). > + */
Broken comment style.
> + atomic_set(&key->enabled, -0xffff); > jump_label_update(key); > /* > * Ensure that if the above cmpxchg loop observes our positive > @@ -158,7 +161,7 @@ void static_key_enable_cpuslocked(struct static_key *key) > > jump_label_lock(); > if (atomic_read(&key->enabled) == 0) { > - atomic_set(&key->enabled, -1); > + atomic_set(&key->enabled, -0xffff); > jump_label_update(key); > /* > * See static_key_slow_inc(). > @@ -208,15 +211,11 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key, > { > lockdep_assert_cpus_held(); > > - /* > - * The negative count check is valid even when a negative > - * key->enabled is in use by static_key_slow_inc(); a > - * __static_key_slow_dec() before the first static_key_slow_inc() > - * returns is unbalanced, because all other static_key_slow_inc() > - * instances block while the update is in progress. > - */ > if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) { > - WARN(atomic_read(&key->enabled) < 0, > + int v; > + > + v = atomic_read(&key->enabled); > + WARN(v < 0 && v != -0xffff, > "jump label: negative count!\n"); > return; > }
> Alternatively we could implement atomic_dec_and_mutex_lock_return().
I think I like that better, something like:
--- kernel/jump_label.c | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/kernel/jump_label.c b/kernel/jump_label.c index bad96b476eb6..a799b1ac6b2f 100644 --- a/kernel/jump_label.c +++ b/kernel/jump_label.c @@ -206,6 +206,8 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key, unsigned long rate_limit, struct delayed_work *work) { + int val; + lockdep_assert_cpus_held(); /* @@ -215,17 +217,20 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key, * returns is unbalanced, because all other static_key_slow_inc() * instances block while the update is in progress. */ - if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) { - WARN(atomic_read(&key->enabled) < 0, - "jump label: negative count!\n"); + val = atomic_fetch_add_unless(&key->enabled, -1, 1); + if (val != 1) { + WARN(val < 0, "jump label: negative count!\n"); return; } - if (rate_limit) { - atomic_inc(&key->enabled); - schedule_delayed_work(work, rate_limit); - } else { - jump_label_update(key); + jump_label_lock(); + if (atomic_dec_and_test(&key->enabled)) { + if (rate_limit) { + atomic_inc(&key->enabled); + schedule_delayed_work(work, rate_limit); + } else { + jump_label_update(key); + } } jump_label_unlock(); }
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