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On Illumos callers of cv_timedwait and cv_timedwait_hires can't distinguish between whether or not the cv was signaled or the call timed out. Illumos handles this (for some definition of handles) by calling cv_signal in the return path if we were signaled but the return value indicates instead that we timed out. This would make sense if it were possible to query the the cv for its net signal disposition. However, this isn't possible and, in spite of the fact that there are places in the code that clearly take a different and incompatible path if a timeout value is indicated, this distinction appears to be rather subtle to most developers. This problem is further compounded by the fact that on Linux, calling cv_signal in the return path wouldn't even do the right thing unless there are other waiters. Since it is possible for the caller to independently determine how much time is remaining but it is not possible to query if the cv was in fact signaled, prioritizing signalling over timeout seems like a cleaner solution. In addition, judging from usage patterns within the code itself, it is also less error prone. Reviewed-by: Jorgen Lundman <lundman@lundman.net> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Alexander Motin <mav@FreeBSD.org> Signed-off-by: Matt Macy <mmacy@FreeBSD.org> Closes #10471
114 lines
4.4 KiB
C
114 lines
4.4 KiB
C
/*
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* Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
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* Copyright (C) 2007 The Regents of the University of California.
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* Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
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* Written by Brian Behlendorf <behlendorf1@llnl.gov>.
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* UCRL-CODE-235197
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*
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* This file is part of the SPL, Solaris Porting Layer.
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* For details, see <http://zfsonlinux.org/>.
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*
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* The SPL is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License as published by the
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* Free Software Foundation; either version 2 of the License, or (at your
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* option) any later version.
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*
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* The SPL is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with the SPL. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef _SPL_CONDVAR_H
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#define _SPL_CONDVAR_H
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#include <linux/module.h>
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#include <sys/kmem.h>
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#include <sys/mutex.h>
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#include <sys/callo.h>
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#include <sys/wait.h>
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#include <sys/time.h>
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/*
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* cv_timedwait() is similar to cv_wait() except that it additionally expects
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* a timeout value specified in ticks. When woken by cv_signal() or
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* cv_broadcast() it returns 1, otherwise when the timeout is reached -1 is
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* returned.
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*
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* cv_timedwait_sig() behaves the same as cv_timedwait() but blocks
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* interruptibly and can be woken by a signal (EINTR, ERESTART). When
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* this occurs 0 is returned.
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*
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* cv_timedwait_io() and cv_timedwait_sig_io() are variants of cv_timedwait()
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* and cv_timedwait_sig() which should be used when waiting for outstanding
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* IO to complete. They are responsible for updating the iowait accounting
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* when this is supported by the platform.
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*
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* cv_timedwait_hires() and cv_timedwait_sig_hires() are high resolution
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* versions of cv_timedwait() and cv_timedwait_sig(). They expect the timeout
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* to be specified as a hrtime_t allowing for timeouts of less than a tick.
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*
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* N.B. The return values differ slightly from the illumos implementation
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* which returns the time remaining, instead of 1, when woken. They both
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* return -1 on timeout. Consumers which need to know the time remaining
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* are responsible for tracking it themselves.
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*/
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/*
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* The kcondvar_t struct is protected by mutex taken externally before
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* calling any of the wait/signal funs, and passed into the wait funs.
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*/
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#define CV_MAGIC 0x346545f4
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#define CV_DESTROY 0x346545f5
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typedef struct {
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int cv_magic;
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spl_wait_queue_head_t cv_event;
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spl_wait_queue_head_t cv_destroy;
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atomic_t cv_refs;
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atomic_t cv_waiters;
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kmutex_t *cv_mutex;
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} kcondvar_t;
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typedef enum { CV_DEFAULT = 0, CV_DRIVER } kcv_type_t;
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extern void __cv_init(kcondvar_t *, char *, kcv_type_t, void *);
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extern void __cv_destroy(kcondvar_t *);
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extern void __cv_wait(kcondvar_t *, kmutex_t *);
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extern void __cv_wait_io(kcondvar_t *, kmutex_t *);
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extern int __cv_wait_io_sig(kcondvar_t *, kmutex_t *);
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extern int __cv_wait_sig(kcondvar_t *, kmutex_t *);
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extern int __cv_timedwait(kcondvar_t *, kmutex_t *, clock_t);
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extern int __cv_timedwait_io(kcondvar_t *, kmutex_t *, clock_t);
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extern int __cv_timedwait_sig(kcondvar_t *, kmutex_t *, clock_t);
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extern int cv_timedwait_hires(kcondvar_t *, kmutex_t *, hrtime_t,
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hrtime_t res, int flag);
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extern int cv_timedwait_sig_hires(kcondvar_t *, kmutex_t *, hrtime_t,
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hrtime_t res, int flag);
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extern void __cv_signal(kcondvar_t *);
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extern void __cv_broadcast(kcondvar_t *c);
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#define cv_init(cvp, name, type, arg) __cv_init(cvp, name, type, arg)
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#define cv_destroy(cvp) __cv_destroy(cvp)
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#define cv_wait(cvp, mp) __cv_wait(cvp, mp)
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#define cv_wait_io(cvp, mp) __cv_wait_io(cvp, mp)
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#define cv_wait_io_sig(cvp, mp) __cv_wait_io_sig(cvp, mp)
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#define cv_wait_sig(cvp, mp) __cv_wait_sig(cvp, mp)
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#define cv_signal(cvp) __cv_signal(cvp)
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#define cv_broadcast(cvp) __cv_broadcast(cvp)
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/*
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* NB: There is no way to reliably distinguish between having been signalled
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* and having timed out on Linux. If the client code needs to reliably
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* distinguish between the two it should use the hires variant.
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*/
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#define cv_timedwait(cvp, mp, t) __cv_timedwait(cvp, mp, t)
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#define cv_timedwait_io(cvp, mp, t) __cv_timedwait_io(cvp, mp, t)
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#define cv_timedwait_sig(cvp, mp, t) __cv_timedwait_sig(cvp, mp, t)
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#endif /* _SPL_CONDVAR_H */
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