Re: [PATCH v14 05/15] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Removing the ARM64-specific `smp_cond_load_acquire_timewait()` fallback before replacing it with a working alternative breaks BPF `rqspinlock` timeout functionality, causing indefinite hangs on platforms without a timer event stream.
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--- Patch [5]: [PATCH v14 05/15] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait() ---
commit e150f6dc634a9b6af6666a67d9bc5ea262bedad7
Author: Ankur Arora <ankur.a.arora@oracle.com>

arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait()

This commit removes the private definition of smp_cond_load_acquire_timewait()
and its helper macros from arch/arm64/include/asm/rqspinlock.h. The commit
message notes this is in preparation for defining
smp_cond_load_acquire_timeout(), allowing rqspinlock to fall back to using
smp_cond_load_acquire() in the interim.

> diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
> index 9ea0a74e58927..a385603436e98 100644
> --- a/arch/arm64/include/asm/rqspinlock.h
> +++ b/arch/arm64/include/asm/rqspinlock.h
[ ... ]
> -#define smp_cond_load_acquire_timewait(ptr, cond_expr,			\
> -				      time_expr_ns, time_limit_ns)	\
> -({									\
> -	__unqual_scalar_typeof(*ptr) _val;				\
> -	int __wfe = arch_timer_evtstrm_available();			\
> -									\
> -	if (likely(__wfe)) {						\
> -		_val = __smp_cond_load_acquire_timewait(ptr, cond_expr,	\
> -							time_expr_ns,	\
> -							time_limit_ns);	\
> -	} else {							\
> -		_val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr,	\
> -							time_expr_ns,	\
> -							time_limit_ns);	\
> -		smp_acquire__after_ctrl_dep();				\
> -	}								\
> -	(typeof(*ptr))_val;						\
> -})
> -
> -#endif
> -
> -#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire_timewait(v, c, 0, 1)

[Severity: High]
With this custom fallback removed, the BPF rqspinlock code will use the generic
smp_cond_load_acquire(). Does this mean on platforms without a timer event
stream (arch_timer_evtstrm_available() is false), a CPU could get permanently
stuck in a WFE instruction during a lock timeout?

In kernel/bpf/rqspinlock.c:resilient_queued_spin_lock_slowpath(), the timeout is
evaluated as part of the condition expression:

    res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret, _Q_LOCKED_MASK));

But smp_cond_load_acquire() calls __cmpwait_relaxed() which executes WFE:

    __cmpwait_relaxed(__PTR, VAL);

If no stores occur to the locked byte, WFE will wait indefinitely, and the
embedded timeout check won't be evaluated while the CPU is asleep.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260714073041.40250-1-ankur.a.arora@oracle.com?part=5