Re: [PATCH bpf-next 02/17] bpf: Use mutex lock pool for bpf trampolines

Adding mutex lock pool that replaces bpf trampolines mutex.

For tracing_multi link coming in following changes we need to lock all
the involved trampolines during the attachment. This could mean thousands
of mutex locks, which is not convenient.

As suggested by Andrii we can replace bpf trampolines mutex with mutex
pool, where each trampoline is hash-ed to one of the locks from the pool.

It's better to lock all the pool mutexes (64 at the moment) than
thousands of them.

Removing the mutex_is_locked in bpf_trampoline_put, because we removed
the mutex from bpf_trampoline.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 include/linux/bpf.h     |  2 --
 kernel/bpf/trampoline.c | 74 +++++++++++++++++++++++++++++++----------
 2 files changed, 56 insertions(+), 20 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index cd9b96434904..46bf3d86bdb2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1335,8 +1335,6 @@ struct bpf_trampoline {
        /* hlist for trampoline_ip_table */
        struct hlist_node hlist_ip;
        struct ftrace_ops *fops;
-       /* serializes access to fields of this trampoline */
-       struct mutex mutex;
        refcount_t refcnt;
        u32 flags;
        u64 key;

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 952cd7932461..05dc0358654d 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c

@@ -30,6 +30,45 @@ static struct hlist_head trampoline_ip_table[TRAMPOLINE_TABLE_SIZE];
 /* serializes access to trampoline tables */
 static DEFINE_MUTEX(trampoline_mutex);

+#define TRAMPOLINE_LOCKS_BITS 6
+#define TRAMPOLINE_LOCKS_TABLE_SIZE (1 << TRAMPOLINE_LOCKS_BITS)
+
+static struct {
+       struct mutex mutex;
+       struct lock_class_key key;
+} *trampoline_locks;
+
+static struct mutex *trampoline_locks_lookup(struct bpf_trampoline *tr)

+{
+       return &trampoline_locks[hash_64((u64) tr, TRAMPOLINE_LOCKS_BITS)].mutex;
+}
+
+static void trampoline_lock(struct bpf_trampoline *tr)
+{
+       mutex_lock(trampoline_locks_lookup(tr));
+}
+
+static void trampoline_unlock(struct bpf_trampoline *tr)
+{
+       mutex_unlock(trampoline_locks_lookup(tr));
+}
+
+static int __init trampoline_locks_init(void)
+{
+       int i;
+
+       trampoline_locks = kmalloc_array(TRAMPOLINE_LOCKS_TABLE_SIZE,
+                                        sizeof(trampoline_locks[0]), GFP_KERNEL);

+       if (!trampoline_locks)
+               return -ENOMEM;
+
+       for (i = 0; i < TRAMPOLINE_LOCKS_TABLE_SIZE; i++) {
+               lockdep_register_key(&trampoline_locks[i].key);