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bpf, sockmap: Fix preempt_rt splat when using raw_spin_lock_t
Sockmap and sockhash maps are a collection of psocks that are
objects representing a socket plus a set of metadata needed
to manage the BPF programs associated with the socket. These
maps use the stab->lock to protect from concurrent operations
on the maps, e.g. trying to insert to objects into the array
at the same time in the same slot. Additionally, a sockhash map
has a bucket lock to protect iteration and insert/delete into
the hash entry.
Each psock has a psock->link which is a linked list of all the
maps that a psock is attached to. This allows a psock (socket)
to be included in multiple sockmap and sockhash maps. This
linked list is protected the psock->link_lock.
They _must_ be nested correctly to avoid deadlock:
lock(stab->lock)
: do BPF map operations and psock insert/delete
lock(psock->link_lock)
: add map to psock linked list of maps
unlock(psock->link_lock)
unlock(stab->lock)
For non PREEMPT_RT kernels both raw_spin_lock_t and spin_lock_t
are guaranteed to not sleep. But, with PREEMPT_RT kernels the
spin_lock_t variants may sleep. In the current code we have
many patterns like this:
rcu_critical_section:
raw_spin_lock(stab->lock)
spin_lock(psock->link_lock) <- may sleep ouch
spin_unlock(psock->link_lock)
raw_spin_unlock(stab->lock)
rcu_critical_section
Nesting spin_lock() inside a raw_spin_lock() violates locking
rules for PREEMPT_RT kernels. And additionally we do alloc(GFP_ATOMICS)
inside the stab->lock, but those might sleep on PREEMPT_RT kernels.
The result is splats like this:
./test_progs -t sockmap_basic
[ 33.344330] bpf_testmod: loading out-of-tree module taints kernel.
[ 33.441933]
[ 33.442089] =============================
[ 33.442421] [ BUG: Invalid wait context ]
[ 33.442763] 6.5.0-rc5-01731-gec0ded2e0282 #4958 Tainted: G O
[ 33.443320] -----------------------------
[ 33.443624] test_progs/2073 is trying to lock:
[ 33.443960] ffff888102a1c290 (&psock->link_lock){....}-{3:3}, at: sock_map_update_common+0x2c2/0x3d0
[ 33.444636] other info that might help us debug this:
[ 33.444991] context-{5:5}
[ 33.445183] 3 locks held by test_progs/2073:
[ 33.445498] #0: ffff88811a208d30 (sk_lock-AF_INET){+.+.}-{0:0}, at: sock_map_update_elem_sys+0xff/0x330
[ 33.446159] #1: ffffffff842539e0 (rcu_read_lock){....}-{1:3}, at: sock_map_update_elem_sys+0xf5/0x330
[ 33.446809] #2: ffff88810d687240 (&stab->lock){+...}-{2:2}, at: sock_map_update_common+0x177/0x3d0
[ 33.447445] stack backtrace:
[ 33.447655] CPU: 10 PID
To fix observe we can't readily remove the allocations (for that
we would need to use/create something similar to bpf_map_alloc). So
convert raw_spin_lock_t to spin_lock_t. We note that sock_map_update
that would trigger the allocate and potential sleep is only allowed
through sys_bpf ops and via sock_ops which precludes hw interrupts
and low level atomic sections in RT preempt kernel. On non RT
preempt kernel there are no changes here and spin locks sections
and alloc(GFP_ATOMIC) are still not sleepable.
Signed-off-by: 
John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230830053517.166611-1-john.fastabend@gmail.com