rqspinlock: Add deadlock detection and recovery

rqspinlock: Add deadlock detection and recovery

While the timeout logic provides guarantees for the waiter's forward
progress, the time until a stalling waiter unblocks can still be long.
The default timeout of 1/4 sec can be excessively long for some use
cases.  Additionally, custom timeouts may exacerbate recovery time.

Introduce logic to detect common cases of deadlocks and perform quicker
recovery. This is done by dividing the time from entry into the locking
slow path until the timeout into intervals of 1 ms. Then, after each
interval elapses, deadlock detection is performed, while also polling
the lock word to ensure we can quickly break out of the detection logic
and proceed with lock acquisition.

A 'held_locks' table is maintained per-CPU where the entry at the bottom
denotes a lock being waited for or already taken. Entries coming before
it denote locks that are already held. The current CPU's table can thus
be looked at to detect AA deadlocks. The tables from other CPUs can be
looked at to discover ABBA situations. Finally, when a matching entry
for the lock being taken on the current CPU is found on some other CPU,
a deadlock situation is detected. This function can take a long time,
therefore the lock word is constantly polled in each loop iteration to
ensure we can preempt detection and proceed with lock acquisition, using
the is_lock_released check.

We set 'spin' member of rqspinlock_timeout struct to 0 to trigger
deadlock checks immediately to perform faster recovery.

Note: Extending lock word size by 4 bytes to record owner CPU can allow
faster detection for ABBA. It is typically the owner which participates
in a ABBA situation. However, to keep compatibility with existing lock
words in the kernel (struct qspinlock), and given deadlocks are a rare
event triggered by bugs, we choose to favor compatibility over faster
detection.

The release_held_lock_entry function requires an smp_wmb, while the
release store on unlock will provide the necessary ordering for us. Add
comments to document the subtleties of why this is correct. It is
possible for stores to be reordered still, but in the context of the
deadlock detection algorithm, a release barrier is sufficient and
needn't be stronger for unlock's case.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-13-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>