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Message-ID: <671056c7-5fc7-60c0-4035-b11d43d95bf1@redhat.com>
Date: Mon, 31 Jul 2017 15:04:06 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>,
torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
tglx@...utronix.de, mingo@...nel.org, dvyukov@...gle.com,
hpa@...or.com
Cc: linux-tip-commits@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [tip:locking/urgent] locking/static_key: Fix concurrent
static_key_slow_inc()
On 31/07/2017 11:36, Peter Zijlstra wrote:
> On Fri, Jun 24, 2016 at 01:59:06AM -0700, tip-bot for Paolo Bonzini wrote:
>> +++ b/kernel/jump_label.c
>> @@ -58,13 +58,36 @@ static void jump_label_update(struct static_key *key);
>>
>> void static_key_slow_inc(struct static_key *key)
>> {
>> + int v, v1;
>> +
>> STATIC_KEY_CHECK_USE();
>> - if (atomic_inc_not_zero(&key->enabled))
>> - return;
>> +
>> + /*
>> + * Careful if we get concurrent static_key_slow_inc() calls;
>> + * later calls must wait for the first one to _finish_ the
>> + * jump_label_update() process. At the same time, however,
>> + * the jump_label_update() call below wants to see
>> + * static_key_enabled(&key) for jumps to be updated properly.
>> + *
>> + * So give a special meaning to negative key->enabled: it sends
>> + * static_key_slow_inc() down the slow path, and it is non-zero
>> + * so it counts as "enabled" in jump_label_update(). Note that
>> + * atomic_inc_unless_negative() checks >= 0, so roll our own.
>> + */
>> + for (v = atomic_read(&key->enabled); v > 0; v = v1) {
>> + v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
>> + if (likely(v1 == v))
>> + return;
>> + }
>>
>> jump_label_lock();
>> - if (atomic_inc_return(&key->enabled) == 1)
>> + if (atomic_read(&key->enabled) == 0) {
>> + atomic_set(&key->enabled, -1);
>> jump_label_update(key);
>> + atomic_set(&key->enabled, 1);
>> + } else {
>> + atomic_inc(&key->enabled);
>> + }
>> jump_label_unlock();
>> }
>
>
> So I was recently looking at this again and got all paranoid. Do we want
> something like so?
Though I agree with the paranoia sentiment, no:
- key->enabled cannot go from 0 to nonzero outside jump_label_mutex.
For this reason the atomic_try_cmpxchg is unnecessary.
- the (implied) smp_mb before jump_label_update is not interesting, but
I don't think it is useful because: 1) during the jump_label_update
there is no correspondence between what static_key_enabled returns and
what the text look like; 2) what would it even be pairing with?
- the smp_mb (though it could be a smp_wmb or atomic_set_release)
initially triggered my paranoia indeed. But even then, I can't see why
you would need it because there's nothing it pairs with. Rather, it's
*any use of key->enabled outside jump_label_lock* (meaning: any use of
static_key_enabled and static_key_count outside the core jump_label.c
code) that should be handled with care.
And indeed, while there aren't many, I think two of them are wrong (and
not fixed by your patch):
- include/linux/cpuset.h defines nr_cpusets which uses static_key_count.
It makes no sense to call it outside cpuset_mutex, and indeed that's
how kernel/cgroup/cpuset.c uses it (nr_cpusets <- generate_sched_domains
<- rebuild_sched_domains_locked). The function should be moved inside
kernel/cgroup/cpuset.c since the mutex is static.
- kernel/cgroup/cgroup.c only enables/disables at init, so using
static_key_enabled should be fine.
- kernel/tracepoint.c only manipulates tracepoint static keys under
tracepoints_mutex, and uses static_key_enabled under the same mutex, so
it's fine.
- net/ipv4/udp.c and net/ipv6/udp.c want to implement a "once-only"
increment of the static key. It's racy and maybe we should provide a
new API static_key_enable_forever:
void static_key_enable_forever(struct static_key *key)
{
STATIC_KEY_CHECK_USE();
if (atomic_read(&key->enabled) > 0)
return;
cpus_read_lock();
jump_label_lock();
if (atomic_read(&key->enabled) == 0) {
atomic_set(&key->enabled, -1);
jump_label_update(key);
atomic_set(&key->enabled, 1);
}
jump_label_unlock();
cpus_read_unlock();
}
EXPORT_SYMBOL_GPL(static_key_enable_forever);
I can prepare a patch if you agree.
Paolo
> ---
> kernel/jump_label.c | 24 +++++++++++++++++-------
> 1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index d11c506a6ac3..69d07e78e48b 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -103,7 +103,7 @@ EXPORT_SYMBOL_GPL(static_key_disable);
>
> void static_key_slow_inc(struct static_key *key)
> {
> - int v, v1;
> + int v;
>
> STATIC_KEY_CHECK_USE();
>
> @@ -119,18 +119,28 @@ void static_key_slow_inc(struct static_key *key)
> * so it counts as "enabled" in jump_label_update(). Note that
> * atomic_inc_unless_negative() checks >= 0, so roll our own.
> */
> - for (v = atomic_read(&key->enabled); v > 0; v = v1) {
> - v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
> - if (likely(v1 == v))
> + for (v = atomic_read(&key->enabled); v > 0;) {
> + if (atomic_try_cmpxchg(&key->enabled, &v, v+1))
> return;
> }
>
> cpus_read_lock();
> jump_label_lock();
> - if (atomic_read(&key->enabled) == 0) {
> - atomic_set(&key->enabled, -1);
> +
> + if (atomic_try_cmpxchg(&key->enabled, 0, -1)) {
> + /*
> + * smp_mb implied, must have -1 before proceeding to change
> + * text.
> + */
> jump_label_update(key);
> - atomic_set(&key->enabled, 1);
> +
> + /*
> + * smp_mb, such that we finish modifying text before enabling
> + * the fast path. Probably not needed because modifying text is
> + * likely to serialize everything. Be paranoid.
> + */
> + smp_mb__before_atomic();
> + atomic_add(2, &key->enabled); /* -1 -> 1 */
> } else {
> atomic_inc(&key->enabled);
> }
>
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