Make kevent(2) a cancellation point.

Note that to cancel blocked kevent(2) call, changelist must be empty,
since we cannot cancel a call which already made changes to the
process state.  And in reverse, call which only makes changes to the
kqueue state, without waiting for an event, is not cancellable.  This
makes a natural usage model to migrate kqueue loop to support
cancellation, where existing single kevent(2) call must be split into
two: first uncancellable update of kqueue, then cancellable wait for
events.

Note that this is ABI-incompatible change, but it is believed that
there is no cancel-safe code that relies on kevent(2) not being a
cancellation point.  Option to preserve the ABI would be to keep
kevent(2) as is, but add new call with flags to specify cancellation
behaviour, which only value seems to add complications.

Suggested and reviewed by:	jilles
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

lib/libc/include/libc_private.h

@@ -221,6 +221,7 @@ enum {
 	INTERPOS__pthread_mutex_init_calloc_cb,
 	INTERPOS_spinlock,
 	INTERPOS_spinunlock,
+	INTERPOS_kevent,
 	INTERPOS_MAX
 };
 
@@ -293,6 +294,7 @@ void *	__sys_freebsd6_mmap(void *, __size_t, int, int, int, int, __off_t);
 struct aiocb;
 struct fd_set;
 struct iovec;
+struct kevent;
 struct msghdr;
 struct pollfd;
 struct rusage;
@@ -315,6 +317,8 @@ int		__sys_fsync(int);
 __pid_t		__sys_fork(void);
 int		__sys_ftruncate(int, __off_t);
 int		__sys_gettimeofday(struct timeval *, struct timezone *);
+int		__sys_kevent(int, const struct kevent *, int, struct kevent *,
+		    int, const struct timespec *);
 __off_t		__sys_lseek(int, __off_t, int);
 void	       *__sys_mmap(void *, __size_t, int, int, int, __off_t);
 int		__sys_msync(void *, __size_t, int);

lib/libc/sys/Makefile.inc

@@ -51,6 +51,7 @@ INTERPOSED = \
 	fcntl \
 	fsync \
 	fork \
+	kevent \
 	msync \
 	nanosleep \
 	open \

lib/libc/sys/interposing_table.c

@@ -75,6 +75,7 @@ interpos_func_t __libc_interposing[INTERPOS_MAX] = {
 	SLOT(_pthread_mutex_init_calloc_cb, _pthread_mutex_init_calloc_cb_stub),
 	SLOT(spinlock, __libc_spinlock_stub),
 	SLOT(spinunlock, __libc_spinunlock_stub),
+	SLOT(kevent, __sys_kevent),
 };
 #undef SLOT
 

lib/libc/sys/kevent.c

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2015 The FreeBSD Foundation.
+ * All rights reserved.
+ *
+ * Portions of this software were developed by Konstantin Belousov
+ * under sponsorship from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice(s), this list of conditions and the following disclaimer as
+ *    the first lines of this file unmodified other than the possible
+ *    addition of one or more copyright notices.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice(s), this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+ * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+ * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+ * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/types.h>
+#include <sys/event.h>
+#include <sys/time.h>
+#include "libc_private.h"
+
+__weak_reference(__sys_kevent, __kevent);
+
+#pragma weak kevent
+int
+kevent(int kq, const struct kevent *changelist, int nchanges,
+    struct kevent *eventlist, int nevents, const struct timespec *timeout)
+{
+
+	return (((int (*)(int, const struct kevent *, int,
+	    struct kevent *, int, const struct timespec *))
+	    __libc_interposing[INTERPOS_kevent])(kq, changelist, nchanges,
+	   eventlist, nevents, timeout));
+}

lib/libc/sys/kqueue.2

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 18, 2014
+.Dd March 29, 2015
 .Dt KQUEUE 2
 .Os
 .Sh NAME
@@ -41,7 +41,7 @@
 .Fn kqueue "void"
 .Ft int
 .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
-.Fn EV_SET "&kev" ident filter flags fflags data udata
+.Fn EV_SET "kev" ident filter flags fflags data udata
 .Sh DESCRIPTION
 The
 .Fn kqueue
@@ -550,6 +550,16 @@ On return,
 .Va fflags
 contains the users defined flags in the lower 24 bits.
 .El
+.Sh CANCELLATION BEHAVIOUR
+If
+.Fa nevents
+is non-zero, i.e. the function is potentially blocking, the call
+is a cancellation point.
+Otherwise, i.e. if
+.Fa nevents
+is zero, the call is not cancellable.
+Cancellation can only occur before any changes are made to the kqueue,
+or when the call was blocked and no changes to the queue were requested.
 .Sh RETURN VALUES
 The
 .Fn kqueue
@@ -620,6 +630,8 @@ The specified descriptor is invalid.
 .It Bq Er EINTR
 A signal was delivered before the timeout expired and before any
 events were placed on the kqueue for return.
+.It Bq Er EINTR
+A cancellation request was delivered to the thread, but not yet handled.
 .It Bq Er EINVAL
 The specified time limit or filter is invalid.
 .It Bq Er ENOENT
@@ -634,6 +646,14 @@ sysctl.
 .It Bq Er ESRCH
 The specified process to attach to does not exist.
 .El
+.Pp
+When
+.Fn kevent
+call fails with
+.Er EINTR
+error, all changes in the
+.Fa changelist
+have been applied.
 .Sh SEE ALSO
 .Xr aio_error 2 ,
 .Xr aio_read 2 ,
@@ -643,6 +663,7 @@ The specified process to attach to does not exist.
 .Xr select 2 ,
 .Xr sigaction 2 ,
 .Xr write 2 ,
+.Xr pthread_setcancelstate 3 ,
 .Xr signal 3
 .Sh HISTORY
 The

lib/libthr/thread/thr_syscalls.c

@@ -341,6 +341,29 @@ __thr_pselect(int count, fd_set *rfds, fd_set *wfds, fd_set *efds,
 	return (ret);
 }
 
+static int
+__thr_kevent(int kq, const struct kevent *changelist, int nchanges,
+    struct kevent *eventlist, int nevents, const struct timespec *timeout)
+{
+	struct pthread *curthread;
+	int ret;
+
+	if (nevents == 0) {
+		/*
+		 * No blocking, do not make the call cancellable.
+		 */
+		return (__sys_kevent(kq, changelist, nchanges, eventlist,
+		    nevents, timeout));
+	}
+	curthread = _get_curthread();
+	_thr_cancel_enter(curthread);
+	ret = __sys_kevent(kq, changelist, nchanges, eventlist, nevents,
+	    timeout);
+	_thr_cancel_leave(curthread, ret == -1 && nchanges == 0);
+
+	return (ret);
+}
+
 /*
  * Cancellation behavior:
  *   Thread may be canceled at start, but if the system call got some data, 
@@ -599,6 +622,7 @@ __thr_interpose_libc(void)
 	SLOT(writev);
 	SLOT(spinlock);
 	SLOT(spinunlock);
+	SLOT(kevent);
 #undef SLOT
 	*(__libc_interposing_slot(
 	    INTERPOS__pthread_mutex_init_calloc_cb)) =

share/man/man3/pthread_testcancel.3

@@ -1,5 +1,5 @@
 .\" $FreeBSD$
-.Dd June 11, 2013
+.Dd March 29, 2015
 .Dt PTHREAD_TESTCANCEL 3
 .Os
 .Sh NAME
@@ -107,6 +107,7 @@ functions:
 .Fn close ,
 .Fn creat ,
 .Fn fsync ,
+.Fn kevent ,
 .Fn mq_receive ,
 .Fn mq_send ,
 .Fn mq_timedreceive ,
@@ -147,12 +148,20 @@ functions:
 .Fn waitpid ,
 .Fn write ,
 .Fn writev .
+.Pp
 The
 .Fn fcntl
 function is a cancellation point if
 .Fa cmd
 is
 .Dv F_SETLKW .
+.Pp
+The
+.Fn kevent
+function is a cancellation point if it is potentially blocking,
+i.e. when the
+.Fa nevents
+argument  is non-zero.
 .Sh RETURN VALUES
 If successful, the
 .Fn pthread_setcancelstate