Re: [PATCH net-next 2/2] net/smc: reduce TX slot contention with exclusive wait

[D. Wythe]

On Tue, May 12, 2026 at 10:26:43AM +0200, Paolo Abeni wrote:
> From: AI Reviewer <ai@xxxxxxxxxxx>
> 
> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
> Full review at: https://netdev-ai.bots.linux.dev/sashiko/
> ---
> net/smc: reduce TX slot contention with exclusive wait
> 
> This patch replaces the non-exclusive wait_event_interruptible_timeout()
> in smc_wr_tx_get_free_slot() with an open-coded exclusive wait loop via
> prepare_to_wait_exclusive(), so wake_up() on a freed TX slot wakes only
> a single waiter instead of the entire wait queue.
> 
> > diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
> > index 48037a3d97a3..0a6f2befb0e2 100644
> > --- a/net/smc/smc_wr.c
> > +++ b/net/smc/smc_wr.c
> > @@ -171,17 +173,27 @@ int smc_wr_tx_get_free_slot(struct smc_link *link,
> >  		if (rc)
> >  			return rc;
> >  	} else {
> > -		rc = wait_event_interruptible_timeout(
> > -			link->wr_tx_wait,
> > -			!smc_link_sendable(link) ||
> > -			lgr->terminating ||
> > -			(smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY),
> > -			SMC_WR_TX_WAIT_FREE_SLOT_TIME);
> > -		if (!rc) {
> > -			/* timeout - terminate link */
> > -			smcr_link_down_cond_sched(link);
> > -			return -EPIPE;
> > +		rc = 0;
> > +		for (;;) {
> > +			prepare_to_wait_exclusive(&link->wr_tx_wait, &wait,
> > +						  TASK_INTERRUPTIBLE);
> > +			if (!smc_link_sendable(link) || lgr->terminating ||
> > +			    smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY)
> > +				break;
> > +			timeout = schedule_timeout(timeout);
> > +			if (!timeout) {
> > +				/* timeout - terminate link */
> > +				smcr_link_down_cond_sched(link);
> > +				break;
> > +			}
> > +			if (signal_pending(current)) {
> > +				rc = -ERESTARTSYS;
> > +				break;
> > +			}
> 
> [High]
> With exclusive waiters, wake_up() on link->wr_tx_wait wakes only one
> task per freed slot. If that waiter is about to exit via the
> signal_pending() branch, can the freed slot be lost to the remaining
> sleepers?
> 
> Consider two exclusive waiters A and B asleep in schedule_timeout():
> 
>   1. smc_wr_tx_put_slot() -> wake_up(&link->wr_tx_wait) is effectively
>      __wake_up(..., nr_exclusive=1).  default_wake_function makes A
>      runnable, autoremove_wake_function pulls A off the queue, and
>      __wake_up_common stops -- B is not woken.
> 
>   2. A signal is delivered to A before it actually runs.  Because A is
>      already TASK_RUNNING, signal_wake_up only sets TIF_SIGPENDING.
> 
>   3. A returns from schedule_timeout() with timeout > 0, so the
>      !timeout branch is skipped.  signal_pending(current) is true, so
>      the loop sets rc = -ERESTARTSYS and breaks without re-testing
>      smc_wr_tx_get_free_slot_index() and without issuing any wake_up()
>      on the way out.
> 
>   4. B stays asleep on the wait queue even though the wr_tx_mask bit
>      for the freed slot is still available.  With no further
>      completions, B sleeps until the full SMC_WR_TX_WAIT_FREE_SLOT_TIME
>      expires, at which point smcr_link_down_cond_sched() tears down an
>      otherwise healthy link.
> 
> The previous wait_event_interruptible_timeout() was non-exclusive, so
> every waiter was woken on each wake_up() and re-evaluated the
> condition, which avoided this class of lost-wakeup.
> 
> Should the signal path re-check the slot condition before returning,
> or issue a wake_up() on exit so another exclusive waiter gets a chance
> at the slot?
> 

Thanks for the review, all three points are valid.

[High] & [Low]: Both resolved by a unified re-check after finish_wait():
if no slot was acquired in the loop (due to signal or timeout),
re-evaluate the condition one final time. If a slot is now available,
claim it and proceed. No wake_up() is needed on exit — the teardown path
already does wake_up_all when the link goes down, and waking the next
waiter when no slot exists is pointless.

> [Medium]
> The commit message says this is a contention optimization and that
> "teardown behavior is unchanged", but does it also change the errno
> returned to callers on signal delivery?
> 
> Before this patch, wait_event_interruptible_timeout() returned
> -ERESTARTSYS on signal; that value is non-zero, so the old if (!rc)
> branch was skipped, control fell through to the trailing
> if (idx == link->wr_tx_cnt) return -EPIPE, and callers saw -EPIPE.
> 
> After the patch, the signal path does:
> 
>     if (signal_pending(current)) {
>         rc = -ERESTARTSYS;
>         break;
>     }
>     ...
>     finish_wait(&link->wr_tx_wait, &wait);
>     if (rc)
>         return rc;
> 
> so -ERESTARTSYS propagates up through smc_cdc_get_free_slot() (which
> only remaps rc to -EPIPE when conn->killed is set),
> smcr_cdc_get_slot_and_msg_send(), smc_cdc_get_slot_and_msg_send() and
> smc_tx_sendmsg(), meaning userspace sees syscall restart / -EINTR
> instead of -EPIPE.
> 
> Note that the sibling smc_wr_reg_send() in the same file explicitly
> converts -ERESTARTSYS to -EINTR.  Should this return-code change be
> called out in the commit message (and possibly split out), given that
> the stated intent of the patch is purely to reduce contention?

Agreed. I'll keep the return code as -EPIPE to match the original behavior, so
this patch remains a pure contention optimization with no semantic
change.


> [Low]
> On timeout expiry the new loop takes the teardown path without
> re-testing whether a slot became available:
> 
> > +			timeout = schedule_timeout(timeout);
> > +			if (!timeout) {
> > +				/* timeout - terminate link */
> > +				smcr_link_down_cond_sched(link);
> > +				break;
> > +			}
> 
> The old wait_event_interruptible_timeout() expansion re-evaluated the
> condition via ___wait_cond_timeout after schedule_timeout() returned
> 0, so a wake_up() racing with timer expiry could still report success
> and let smc_wr_tx_get_free_slot_index() claim the slot.
> 
> In the narrow race where a slot is freed at the same instant the
> timer fires, does this now tear the link down even though a slot was
> actually available?
> 
> >  		}
> > +		finish_wait(&link->wr_tx_wait, &wait);
> > +		if (rc)
> > +			return rc;
> >  		if (idx == link->wr_tx_cnt)
> >  			return -EPIPE;
> >  	}

D. Wythe

> -- 
> This is an AI-generated review.