Date: Thu, 5 Apr 2001 18:00:57 -0400 (EDT) From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> To: John Baldwin <jhb@FreeBSD.ORG> Cc: current@FreeBSD.ORG Subject: RE: selwakeup() Message-ID: <200104052200.SAA71985@khavrinen.lcs.mit.edu> In-Reply-To: <XFMail.010405144129.jhb@FreeBSD.org> References: <XFMail.010405104534.jhb@FreeBSD.org> <XFMail.010405144129.jhb@FreeBSD.org>
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<<On Thu, 05 Apr 2001 14:41:29 -0700 (PDT), John Baldwin <jhb@FreeBSD.ORG> said: > As a safety check we should probably zero the pid right before zfree()'ing a > proc in wait() however, so that a stale pointer to a free'd process doesn't > have a valid pid if we do this. Should not be necessary. Here is the logic: p = sip->si_p; mtx_lock_spin(&sched_lock); if (p->p_stat != SZOMB || p->p_pid != sip->si_pid) { /* oops */ mtx_lock_spin(&sched_lock); return; } sip->si_pid = 0; sip->si_p = 0; if (p->p_wchan == (caddr_t)&selwait) { /* ... */ If `p' is a pointer to a freed process, then p->p_stat is guaranteed to be SZOMB -- the only code path which can free a process struct is wrapped inside `if (p->p_stat == SZOMB)'. (See kern_exit.c:exit1().) If `p' is a pointer to an active process, and it's the wrong pid, then we don't wake it up. Otherwise, we wake it up. (`p' might still be the wrong process, if pid space wrapped around, but the current code doesn't deal with that condition, either, nor should it.) -GAWollman To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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