6.1-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
[ Upstream commit 1d7f856c2ca449f04a22d876e36b464b7a9d28b6 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/jump_label.c | 34 +++++++++++++++++++++++++++------- 1 file changed, 27 insertions(+), 7 deletions(-)
diff --git a/kernel/jump_label.c b/kernel/jump_label.c index 7374053bbe049..554e04b25b13a 100644 --- a/kernel/jump_label.c +++ b/kernel/jump_label.c @@ -165,7 +165,7 @@ 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); @@ -247,7 +247,7 @@ 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;
@@ -271,6 +271,14 @@ 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))); @@ -281,15 +289,27 @@ 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) @@ -326,7 +346,7 @@ 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);