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is sent to a process, psignal() needs to schedule an AST for the process if the process is runnable, not just if it is current, so that pending signals get checked for on the next return of the process to user mode. This wasn't practical until recently because the AST flag was per-cpu so setting it for a non-current process would usually just cause a bogus AST for the current process. For non-current processes looping in user mode, it took accidental (?) magic to deliver signals at all. Signals were usually delivered late as a side effect of rescheduling (need_resched() sets astpending, etc.). In pre-SMPng, delivery was delayed by at most 1 quantum (the need_resched() call in roundrobin() is certain to occur within 1 quantum for looping processes). In -current, things are complicated by normal interrupt handlers being threads. Missing handling of the complications makes roundrobin() a bogus no-op, but preemptive scheduling sort of works anyway due to even larger bogons elsewhere.
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