ALSA: seq: Use RCU for the client table

The sequencer keeps a global table of clients (clienttab[]) indexed by
client id, protected by the global clients_lock spinlock.  The lookup
snd_seq_client_use_ptr() reads a slot and takes a use_lock reference on
the client, and this runs on the event delivery hot path: every
dispatched event resolves its destination (and often source) client
through it.  The spinlock's only job on the read side is to make the
"pointer is non-NULL" test and the reference increment indivisible with
respect to the writer that nulls the slot and then drains the refcount.

Clients come and go rarely but delivery happens constantly, so this is
yet another read-mostly case as the port and subscriber lists.
Convert the table to RCU: the read side now runs lock-free under
rcu_read_lock() and takes the use_lock reference via
rcu_dereference(), removing contention on the single global spinlock
from the delivery path.  The writers keep clients_lock (still needed
to serialize slot allocation) and publish / unpublish via
rcu_assign_pointer(); creation and destruction remain serialized at a
higher level by register_mutex.

As with the ports, the client is not freed via kfree_rcu(): its lifetime
is governed by the use_lock refcount drained in seq_free_client1().
list_del under the old spinlock excluded a concurrent lookup from taking
a new reference once the slot was nulled; rcu_assign_pointer(NULL) offers
no such exclusion, so a reader still holding the old pointer can grab a
reference after the unpublish.  seq_free_client1() therefore calls
synchronize_rcu() after nulling the slot and before snd_use_lock_sync():
once the grace period elapses no new reference can appear, and the
existing drain then frees the client safely.

clienttablock[] keeps its slot-reservation role (create/free are
serialized by register_mutex); its read on the lookup path only gates
module autoload, so a lockless read is harmless.  Dropping the spinlock
from the read path is safe: clients_lock is now taken only by the
process-context writers, and the sole atomic reader uses RCU, which is
IRQ-safe.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260810133711.42483-4-tiwai@suse.de

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