Re: [PATCH bpf-next v2 00/26] Resilient Queued Spin Lock

Peter Zijlstra
2025-02-10T10:49:39Z

On Mon, Feb 10, 2025 at 10:38:41AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 06, 2025 at 02:54:08AM -0800, Kumar Kartikeya Dwivedi wrote:
>
>
> > Deadlock Detection
> > ~~~~~~~~~~~~~~~~~~
> > We handle two cases of deadlocks: AA deadlocks (attempts to acquire the
> > same lock again), and ABBA deadlocks (attempts to acquire two locks in
> > the opposite order from two distinct threads). Variants of ABBA
> > deadlocks may be encountered with more than two locks being held in the
> > incorrect order. These are not diagnosed explicitly, as they reduce to
> > ABBA deadlocks.
> >
> > Deadlock detection is triggered immediately when beginning the waiting
> > loop of a lock slow path.
> >
> > While timeouts ensure that any waiting loops in the locking slow path
> > terminate and return to the caller, it can be excessively long in some
> > situations. While the default timeout is short (0.5s), a stall for this
> > duration inside the kernel can set off alerts for latency-critical
> > services with strict SLOs. Ideally, the kernel should recover from an
> > undesired state of the lock as soon as possible.
> >
> > A multi-step strategy is used to recover the kernel from waiting loops
> > in the locking algorithm which may fail to terminate in a bounded amount
> > of time.
> >
> > * Each CPU maintains a table of held locks. Entries are inserted and
> > removed upon entry into lock, and exit from unlock, respectively.
> > * Deadlock detection for AA locks is thus simple: we have an AA
> > deadlock if we find a held lock entry for the lock we’re attempting
> > to acquire on the same CPU.
> > * During deadlock detection for ABBA, we search through the tables of
> > all other CPUs to find situations where we are holding a lock the
> > remote CPU is attempting to acquire, and they are holding a lock we
> > are attempting to acquire. Upon encountering such a condition, we
> > report an ABBA deadlock.
> > * We divide the duration between entry time point into the waiting loop
> > and the timeout time point into intervals of 1 ms, and perform
> > deadlock detection until timeout happens. Upon entry into the slow
> > path, and then completion of each 1 ms interval, we perform detection
> > of both AA and ABBA deadlocks. In the event that deadlock detection
> > yields a positive result, the recovery happens sooner than the
> > timeout. Otherwise, it happens as a last resort upon completion of
> > the timeout.
> >
> > Timeouts
> > ~~~~~~~~
> > Timeouts act as final line of defense against stalls for waiting loops.
> > The ‘ktime_get_mono_fast_ns’ function is used to poll for the current
> > time, and it is compared to the timestamp indicating the end time in the
> > waiter loop. Each waiting loop is instrumented to check an extra
> > condition using a macro. Internally, the macro implementation amortizes
> > the checking of the timeout to avoid sampling the clock in every
> > iteration. Precisely, the timeout checks are invoked every 64k
> > iterations.
> >
> > Recovery
> > ~~~~~~~~
>
> I'm probably bad at reading, but I failed to find anything that
> explained how you recover from a deadlock.
>
> Do you force unload the BPF program?

Even the simple AB-BA case,

CPU0 CPU1
lock-A lock-B
lock-B lock-A <-

just having a random lock op return -ETIMO doesn't actually solve
anything. Suppose CPU1's lock-A will time out; it will have to unwind
and release lock-B before CPU0 can make progress.

Worse, if CPU1 isn't quick enough to unwind and release B, then CPU0's
lock-B will also time out.

At which point they'll both try again and you're stuck in the same
place, no?

Given you *have* to unwind to make progress; why not move the entire
thing to a wound-wait style lock? Then you also get rid of the whole
timeout mess.