On Sun, Feb 22, 2026 at 6:34 AM Jiri Olsa <[email protected]> wrote:
>
> On Fri, Feb 20, 2026 at 11:58:13AM -0800, Alexei Starovoitov wrote:
> > On Fri, Feb 20, 2026 at 2:07 AM Jiri Olsa <[email protected]> wrote:
> > >
> > > 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 <[email protected]>
> > > Signed-off-by: Jiri Olsa <[email protected]>
> > > ---
> > > 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)
> >
> > select_trampoline_lock() ?
>
> ok
>
> >
> > > +{
> > > + 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);
> >
> > why bother with memory allocation? This is just 64 mutexes.
>
> ok, I could probably use __mutex_init directly for static key
>
> about 64.. not sure how I missed that but there's lockdep limit for
> maximum locks depth and it's 48.. so we'll need to use 32 locks,
> which is probably still ok
>
> >
> > > + if (!trampoline_locks)
> > > + return -ENOMEM;
> > > +
> > > + for (i = 0; i < TRAMPOLINE_LOCKS_TABLE_SIZE; i++) {
> > > + lockdep_register_key(&trampoline_locks[i].key);
> >
> > why special key?
>
> if we keep single key we will get lockdep 'recursive locking' warning
> during bpf_trampoline_multi_attach, because lockdep will think we lock
> the same mutex
>
> there's support to annotate nested locking with mutex_lock_nested but
> it allows maximum of 8 nested instances
yeah. subclass limit of 8 is there for a different use case.
I guess you never validated your earlier approach of "let's take
all trampoline mutexes" with lockdep ? ;)
MAX_LOCK_DEPTH is indeed 48.
See fs/configfs/inode.c and default_group_class.
It does:
lockdep_set_class(&inode->i_rwsem,
&default_group_class[depth - 1]);
the idea here is that the number of lockdep keys doesn't have
to be equal to the actual number of mutexes.
I guess we can keep a total of 32 mutexes to avoid making it too fancy.
Please add a comment explaining 32 and why it needs lockdep_key.
I thought declaring all mutexes as static will avoid the need for the key,
but DEFINE_MUTEX doesn't support an array.
So since we need a loop anyway to init mutex and the key,
let's keep kmalloc_array() above. Which is now renamed to kmalloc_objs()
after 7.0-rc1.
