Work around deadlock between fstchg and fstcnt
When suspending a filesystem in fstrans_setstate(), we wait on fstcnt for threads to finish transactions. While we do this, any thread trying to start a filesystem transaction will wait on fstchg in fstrans_start(), a situation which can deadlock. The wait for fstcnt in fstrans_setstate() can be interrupted by a signal, but the wait for fstchg in fstrans_start() cannot. Once most processes are stuck in fstchg, it is impossible to send a signal to the thread that waits on fstcnt, because no process respond anymore to user input. We fix that by adding a timeout to the wait on fstcnt in fstrans_setstate(). This means suspending a filesystem may fail, but it was already the case when the sleep was interupted by a signal, hence calling function must already handle a possible failure. Fixes kern/53624
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parent
d794b9b637
commit
83a99212b0
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@ -1,4 +1,4 @@
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/* $NetBSD: vfs_trans.c,v 1.48 2017/06/18 14:00:17 hannken Exp $ */
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/* $NetBSD: vfs_trans.c,v 1.49 2018/09/27 01:03:40 manu Exp $ */
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/*-
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* Copyright (c) 2007 The NetBSD Foundation, Inc.
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@ -30,7 +30,7 @@
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*/
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#include <sys/cdefs.h>
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__KERNEL_RCSID(0, "$NetBSD: vfs_trans.c,v 1.48 2017/06/18 14:00:17 hannken Exp $");
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__KERNEL_RCSID(0, "$NetBSD: vfs_trans.c,v 1.49 2018/09/27 01:03:40 manu Exp $");
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/*
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* File system transaction operations.
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@ -42,6 +42,7 @@ __KERNEL_RCSID(0, "$NetBSD: vfs_trans.c,v 1.48 2017/06/18 14:00:17 hannken Exp $
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#include <sys/param.h>
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#include <sys/systm.h>
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#include <sys/kernel.h>
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#include <sys/atomic.h>
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#include <sys/buf.h>
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#include <sys/kmem.h>
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@ -532,10 +533,14 @@ fstrans_setstate(struct mount *mp, enum fstrans_state new_state)
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/*
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* All threads see the new state now.
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* Wait for transactions invalid at this state to leave.
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* We cannot wait forever because many processes would
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* get stuck waiting for fstcnt in fstrans_start(). This
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* is acute when suspending the root filesystem.
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*/
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error = 0;
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while (! state_change_done(mp)) {
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error = cv_wait_sig(&fstrans_count_cv, &fstrans_lock);
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error = cv_timedwait_sig(&fstrans_count_cv,
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&fstrans_lock, hz / 4);
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if (error) {
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new_state = fmi->fmi_state = FSTRANS_NORMAL;
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break;
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