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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. A private copy of this interface was committed to the tree prematurely. Removal (while temporarily breaking bisection) was acked by the author here: https://lore.kernel.org/lkml/CAP01T75VaNGk+adULuX3q_8uxc0z78J5B_5ECLy2VR=i7pBi0g@mail.gmail.com/ Ankur > > --- Patch [5]: [PATCH v14 05/15] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait() --- > commit e150f6dc634a9b6af6666a67d9bc5ea262bedad7 > Author: Ankur Arora > > 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.