Re: [PATCH can v2] can: j1939: avoid address-claim timer deadlock

[Oleksij Rempel]

Hi Felix,

Please send next patch stand alone, not in replay to the previous patch
or discussion.

On Mon, Aug 03, 2026 at 12:15:03PM +0200, Felix Hoffmann wrote:
> j1939_ac_process() holds priv->lock while synchronously canceling an
> ECU's address-claim hrtimer. The timer callback takes the same lock. If the
> callback starts on another CPU after the receive path takes the lock, the
> callback waits for priv->lock while hrtimer_cancel() waits for the callback
> to finish. This deadlocks both CPUs and makes the system unresponsive.
> 
> Replace the callback's blocking lock acquisition with write_trylock(). If
> the lock is busy, move the expiry forward and restart the timer. This lets
> a concurrent hrtimer_cancel() observe the callback finish and remove the
> requeued timer. Without a cancellation, address mapping is retried shortly.
> 
> The failed write trylock does not necessarily mean that the address-claim
> writer owns priv->lock. Receive processing also takes its read side for
> other frames, and an active reader prevents write_trylock() from
> succeeding. Document both the address-claim writer and receive-path readers
> as possible sources of contention.
> 
> Name the 1 ms retry interval J1939_ECU_TIMER_RETRY_DELAY_MS instead of
> using a literal in the callback. Track consecutive failures per ECU, reset
> the counter when its timer starts, and emit a warning guarded by
> net_ratelimit() on the fifth retry. Five is a pragmatic threshold for
> detecting unusual delay; it is not specified by J1939.
> 
> The timer uses HRTIMER_MODE_REL_SOFT, so its callback runs from
> HRTIMER_SOFTIRQ with bottom halves already disabled. write_trylock() does
> not change bottom-half state and must therefore pair with write_unlock() on
> success. The write_lock_bh()/write_unlock_bh() pair is unnecessary in this
> context.
> 
> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Codex:GPT5.6-Sol
> Signed-off-by: Felix Hoffmann <f3lix.dev@xxxxxx>

Acked-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>

Thank you!

Best Regards,
Oleksij
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