rqspinlock: Choose trylock fallback for NMI waiters

Currently, out of all 3 types of waiters in the rqspinlock slow path
(i.e., pending bit waiter, wait queue head waiter, and wait queue
non-head waiter), only the pending bit waiter and wait queue head
waiters apply deadlock checks and a timeout on their waiting loop. The
assumption here was that the wait queue head's forward progress would be
sufficient to identify cases where the lock owner or pending bit waiter
is stuck, and non-head waiters relying on the head waiter would prove to
be sufficient for their own forward progress.

However, the head waiter itself can be preempted by a non-head waiter
for the same lock (AA) or a different lock (ABBA) in a manner that
impedes its forward progress. In such a case, non-head waiters not
performing deadlock and timeout checks becomes insufficient, and the
system can enter a state of lockup.

This is typically not a concern with non-NMI lock acquisitions, as lock
holders which in run in different contexts (IRQ, non-IRQ) use "irqsave"
variants of the lock APIs, which naturally excludes such lock holders
from preempting one another on the same CPU.

It might seem likely that a similar case may occur for rqspinlock when
programs are attached to contention tracepoints (begin, end), however,
these tracepoints either precede the enqueue into the wait queue, or
succeed it, therefore cannot be used to preempt a head waiter's waiting
loop.

We must still be careful against nested kprobe and fentry programs that
may attach to the middle of the head's waiting loop to stall forward
progress and invoke another rqspinlock acquisition that proceeds as a
non-head waiter. To this end, drop CC_FLAGS_FTRACE from the rqspinlock.o
object file.

For now, this issue is resolved by falling back to a repeated trylock on
the lock word from NMI context, while performing the deadlock checks to
break out early in case forward progress is impossible, and use the
timeout as a final fallback.

A more involved fix to terminate the queue when such a condition occurs
will be made as a follow up. A selftest to stress this aspect of nested
NMI/non-NMI locking attempts will be added in a subsequent patch to the
bpf-next tree when this fix lands and trees are synchronized.

Reported-by: Josef Bacik <josef@toxicpanda.com>
Fixes: 164c246571e9 ("rqspinlock: Protect waiters in queue from stalls")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250909184959.3509085-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index 269c04a24664000ef73226ddb033bc4d9dc49248..f6cf8c2af5f7b3ebb20d8ffff2af163da7655fa4 100644 (file)
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -62,3 +62,4 @@ CFLAGS_REMOVE_bpf_lru_list.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_queue_stack_maps.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_lpm_trie.o = $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_ringbuf.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_rqspinlock.o = $(CC_FLAGS_FTRACE)

diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index 5ab354d55d829566d4e885bca657ed395a2aa6a5..a00561b1d3e515c2d3390eb5ee840000f806b668 100644 (file)
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -471,7 +471,7 @@ queue:
         * any MCS node. This is not the most elegant solution, but is
         * simple enough.
         */
-       if (unlikely(idx >= _Q_MAX_NODES)) {
+       if (unlikely(idx >= _Q_MAX_NODES || in_nmi())) {
                lockevent_inc(lock_no_node);
                RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT);
                while (!queued_spin_trylock(lock)) {