Date: Tue, 28 Jul 2020 07:05:49 +0000 (UTC) From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org Subject: svn commit: r363634 - stable/12/sys/kern Message-ID: <202007280705.06S75nUG094370@repo.freebsd.org>
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Author: mjg Date: Tue Jul 28 07:05:49 2020 New Revision: 363634 URL: https://svnweb.freebsd.org/changeset/base/363634 Log: MFC r363511: Do a lockless check in kthread_suspend_check Modified: stable/12/sys/kern/kern_kthread.c Directory Properties: stable/12/ (props changed) Modified: stable/12/sys/kern/kern_kthread.c ============================================================================== --- stable/12/sys/kern/kern_kthread.c Tue Jul 28 07:04:46 2020 (r363633) +++ stable/12/sys/kern/kern_kthread.c Tue Jul 28 07:05:49 2020 (r363634) @@ -444,12 +444,15 @@ kthread_suspend_check(void) panic("%s: curthread is not a valid kthread", __func__); /* - * As long as the double-lock protection is used when accessing the - * TDF_KTH_SUSP flag, synchronizing the read operation via proc mutex - * is fine. + * Setting the TDF_KTH_SUSP flag is protected by process lock. + * + * Do an unlocked read first to avoid serializing with all other threads + * in the common case of not suspending. */ + if ((td->td_flags & TDF_KTH_SUSP) == 0) + return; PROC_LOCK(p); - while (td->td_flags & TDF_KTH_SUSP) { + while ((td->td_flags & TDF_KTH_SUSP) != 0) { wakeup(&td->td_flags); msleep(&td->td_flags, &p->p_mtx, PPAUSE, "ktsusp", 0); }
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