Commit Graph

170 Commits

Author SHA1 Message Date
Konstantin Belousov
478ca4b004 Rename umtxq_check_susp() to thread_check_susp()
and make it usable outside of kern_umtx.c.  To be used in several
future changes.

Discussed with:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2020-01-02 22:13:59 +00:00
Konstantin Belousov
8f4d74eb1e Style: remove trailing spaces/tabs.
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2020-01-02 22:07:03 +00:00
Konstantin Belousov
5cbdd18fd4 Make umtxq_check_susp() to correctly handle thread exit requests.
The check for P_SINGLE_EXIT was shadowed by the (P_SHOULDSTOP || traced) check.

Reported by:	bdrewery (might be)
Reviewed by:	markj
Tested by:	pho
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D21124
2019-08-01 14:34:27 +00:00
Konstantin Belousov
fd336e2ac0 Fix handling of transient casueword(9) failures in do_sem_wait().
In particular, restart should be only done when the failure is
transient.  For this, recheck the count1 value after the operation.

Note that do_sem_wait() is older usem interface.

Reported and tested by:	bdrewery
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2019-07-31 19:16:49 +00:00
Konstantin Belousov
9f7b7da5bf In do_sem2_wait(), balance umtx_key_get() with umtx_key_release() on retry.
Reported by:	ler
Bisected and reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	12 days
2019-07-15 19:18:25 +00:00
Konstantin Belousov
ad038195bd In do_lock_pi(), do not return prematurely.
If umtxq_check_susp() indicates an exit, we should clean the resources
before returning.  Do it by breaking out of the loop and relying on
post-loop cleanup.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	12 days
Differential revision:	https://reviews.freebsd.org/D20949
2019-07-15 08:39:52 +00:00
Konstantin Belousov
40bd868ba7 Correctly check for casueword(9) success in do_set_ceiling().
After r349951, the return code must be checked instead of old == new
comparision.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	12 days
Differential revision:	https://reviews.freebsd.org/D20949
2019-07-15 08:38:01 +00:00
Konstantin Belousov
30b3018d48 Provide protection against starvation of the ll/sc loops when accessing userpace.
Casueword(9) on ll/sc architectures must be prepared for userspace
constantly modifying the same cache line as containing the CAS word,
and not loop infinitely.  Otherwise, rogue userspace livelocks the
kernel.

To fix the issue, change casueword(9) interface to return new value 1
indicating that either comparision or store failed, instead of relying
on the oldval == *oldvalp comparison.  The primitive no longer retries
the operation if it failed spuriously.  Modify callers of
casueword(9), all in kern_umtx.c, to handle retries, and react to
stops and requests to terminate between retries.

On x86, despite cmpxchg should not return spurious failures, we can
take advantage of the new interface and just return PSL.ZF.

Reviewed by:	andrew (arm64, previous version), markj
Tested by:	pho
Reported by:	https://xenbits.xen.org/xsa/advisory-295.txt
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D20772
2019-07-12 18:43:24 +00:00
Konstantin Belousov
7fde3c6b28 More style.
Re-wrap long lines, reformat comments, remove excessive blank line.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
2019-07-02 21:03:06 +00:00
Konstantin Belousov
4b8b28e130 Style.
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
2019-07-02 19:32:48 +00:00
Konstantin Belousov
2d7a555294 Style.
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
2019-06-28 20:40:54 +00:00
Mateusz Guzik
ac97da9ad8 Reduce umtx-related work on exec and exit
- there is no need to take the process lock to iterate the thread
  list after single-threading is enforced
- typically there are no mutexes to clean up (testable without taking
  the global umtx lock)
- typically there is no need to adjust the priority (testable without
  taking thread lock)

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20160
2019-05-08 16:30:38 +00:00
Mateusz Guzik
6017827676 umtx: avoid umtxshm locking on object termination if possible
Sample build world result on tmpfs:
kern.ipc.umtx_terminate_notempty: 0
kern.ipc.umtx_terminate_empty: 2891815

Sponsored by:	The FreeBSD Foundation
2018-12-08 14:04:57 +00:00
Brooks Davis
b34f4419fb Make freebsd32_umtx_op follow the freebsd32_foo convention.
Sponsored by:	DARPA, AFRL
2018-11-09 00:46:10 +00:00
Alan Somers
6040822c4e Make timespecadd(3) and friends public
The timespecadd(3) family of macros were imported from NetBSD back in
r35029. However, they were initially guarded by #ifdef _KERNEL. In the
meantime, we have grown at least 28 syscalls that use timespecs in some
way, leading many programs both inside and outside of the base system to
redefine those macros. It's better just to make the definitions public.

Our kernel currently defines two-argument versions of timespecadd and
timespecsub.  NetBSD, OpenBSD, and FreeDesktop.org's libbsd, however, define
three-argument versions.  Solaris also defines a three-argument version, but
only in its kernel.  This revision changes our definition to match the
common three-argument version.

Bump _FreeBSD_version due to the breaking KPI change.

Discussed with:	cem, jilles, ian, bde
Differential Revision:	https://reviews.freebsd.org/D14725
2018-07-30 15:46:40 +00:00
Matt Macy
e1a92f058f umtx: don't call umtxq_getchain unless the value is needed 2018-05-19 05:09:10 +00:00
Brooks Davis
6469bdcdb6 Move most of the contents of opt_compat.h to opt_global.h.
opt_compat.h is mentioned in nearly 180 files. In-progress network
driver compabibility improvements may add over 100 more so this is
closer to "just about everywhere" than "only some files" per the
guidance in sys/conf/options.

Keep COMPAT_LINUX32 in opt_compat.h as it is confined to a subset of
sys/compat/linux/*.c.  A fake _COMPAT_LINUX option ensure opt_compat.h
is created on all architectures.

Move COMPAT_LINUXKPI to opt_dontuse.h as it is only used to control the
set of compiled files.

Reviewed by:	kib, cem, jhb, jtl
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14941
2018-04-06 17:35:35 +00:00
Brooks Davis
91a743004c Use umtx_copyin_umtx_time32() in __umtx_op_lock_umutex_compat32().
Non-NULL timeouts where copied in improperly and could produce failures
due to incompatible data structures.

Reviewed by:	kib
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14587
2018-03-06 01:52:04 +00:00
Pedro F. Giffuni
8a36da99de sys/kern: adoption of SPDX licensing ID tags.
Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
2017-11-27 15:20:12 +00:00
Konstantin Belousov
30c438723d Convert explicit panic() call to assert.
Based on github pull request:	#113
Submitted by:	pmarillo@github
MFC after:	1 week
2017-11-04 10:49:34 +00:00
Eric van Gyzen
9dbdf2a169 When the RTC is adjusted, reevaluate absolute sleep times based on the RTC
POSIX 2008 says this about clock_settime(2):

    If the value of the CLOCK_REALTIME clock is set via clock_settime(),
    the new value of the clock shall be used to determine the time
    of expiration for absolute time services based upon the
    CLOCK_REALTIME clock.  This applies to the time at which armed
    absolute timers expire.  If the absolute time requested at the
    invocation of such a time service is before the new value of
    the clock, the time service shall expire immediately as if the
    clock had reached the requested time normally.

    Setting the value of the CLOCK_REALTIME clock via clock_settime()
    shall have no effect on threads that are blocked waiting for
    a relative time service based upon this clock, including the
    nanosleep() function; nor on the expiration of relative timers
    based upon this clock.  Consequently, these time services shall
    expire when the requested relative interval elapses, independently
    of the new or old value of the clock.

When the real-time clock is adjusted, such as by clock_settime(3),
wake any threads sleeping until an absolute real-clock time.
Such a sleep is indicated by a non-zero td_rtcgen.  The sleep functions
will set that field to zero and return zero to tell the caller
to reevaluate its sleep duration based on the new value of the clock.

At present, this affects the following functions:

    pthread_cond_timedwait(3)
    pthread_mutex_timedlock(3)
    pthread_rwlock_timedrdlock(3)
    pthread_rwlock_timedwrlock(3)
    sem_timedwait(3)
    sem_clockwait_np(3)

I'm working on adding clock_nanosleep(2), which will also be affected.

Reported by:	Sebastian Huber <sebastian.huber@embedded-brains.de>
Reviewed by:	jhb, kib
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D9791
2017-03-14 19:06:44 +00:00
Eric van Gyzen
b215ceaaec Add sem_clockwait_np()
This function allows the caller to specify the reference clock
and choose between absolute and relative mode.  In relative mode,
the remaining time can be returned.

The API is similar to clock_nanosleep(3).  Thanks to Ed Schouten
for that suggestion.

While I'm here, reduce the sleep time in the semaphore "child"
test to greatly reduce its runtime.  Also add a reasonable timeout.

Reviewed by:	ed (userland)
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D9656
2017-02-23 19:36:38 +00:00
Adrian Chadd
0046bef85a [mips] make UMTX_CHAINS configurable at compile time.
The default (512) wastes quite a bit of space which doesn't really buy
us much on highly embedded systems which don't take a lot of locks in
parallel.

This makes it at least build time configurable so people can experiment.
2016-11-15 01:34:38 +00:00
Konstantin Belousov
0f2d97838d In both do_rw_wrlock() and do_rw_rdlock() after r304808, do not
obliterate possible error from sleep with errors from
umtxq_check_susp(), when looping to clear URWLOCK_{READ,WRITE}_WAITERS.

Noted and reviewed by:	vangyzen
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2016-08-25 19:15:02 +00:00
Konstantin Belousov
28e21133f3 Prevent leak of URWLOCK_READ_WAITERS flag for urwlocks.
If there was some error, e.g. the sleep was interrupted, as in the
referenced PR, do_rw_rdlock() did not cleared URWLOCK_READ_WAITERS.
Since unlock only wakes up write waiters when there is no read
waiters, for URWLOCK_PREFER_READER kind of locks, the result was
missed wakeups for writers.

In particular, the most visible victims are ld-elf.so locks in
processes which loaded libthr, because rtld locks are urwlocks in
prefer-reader mode.  Normal rwlocks fall into prefer-reader mode only
if thread already owns rw lock in read mode, which is not typical and
correspondingly less visible.  In the PR, unowned rtld bind lock was
waited for in the process where only one thread was left alive.

Note that do_rw_wrlock() correctly clears URWLOCK_WRITE_WAITERS in
case of errors.

Reported and tested by:	longwitz@incore.de
PR:	211947
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2016-08-25 16:35:42 +00:00
Eric Badger
b0f2185bbe sem_post(): wake up the sleeper only after adjusting has_waiters
If the caller of sem_post() wakes up a thread sleeping via sem_wait()
before it clears the has_waiters flag, the caller of sem_wait() has no way of
knowing when it is safe to destroy the semaphore and reuse the memory. This is
because the caller of sem_post() may be interrupted between the wake step and
the clearing of has_waiters. It will then write into the has_waiters flag in
userspace after being preempted for some unknown amount of time.

Reviewed by:	jhb, kib, vangyzen
Approved by:	kib (mentor), vangyzen (mentor)
MFC after:	2 weeks
Sponsored by:	Dell Inc.
Differential Revision:	https://reviews.freebsd.org/D7505
2016-08-15 20:09:09 +00:00
Konstantin Belousov
2a339d9e3d Add implementation of robust mutexes, hopefully close enough to the
intention of the POSIX IEEE Std 1003.1TM-2008/Cor 1-2013.

A robust mutex is guaranteed to be cleared by the system upon either
thread or process owner termination while the mutex is held.  The next
mutex locker is then notified about inconsistent mutex state and can
execute (or abandon) corrective actions.

The patch mostly consists of small changes here and there, adding
neccessary checks for the inconsistent and abandoned conditions into
existing paths.  Additionally, the thread exit handler was extended to
iterate over the userspace-maintained list of owned robust mutexes,
unlocking and marking as terminated each of them.

The list of owned robust mutexes cannot be maintained atomically
synchronous with the mutex lock state (it is possible in kernel, but
is too expensive).  Instead, for the duration of lock or unlock
operation, the current mutex is remembered in a special slot that is
also checked by the kernel at thread termination.

Kernel must be aware about the per-thread location of the heads of
robust mutex lists and the current active mutex slot.  When a thread
touches a robust mutex for the first time, a new umtx op syscall is
issued which informs about location of lists heads.

The umtx sleep queues for PP and PI mutexes are split between
non-robust and robust.

Somewhat unrelated changes in the patch:
1. Style.
2. The fix for proper tdfind() call use in umtxq_sleep_pi() for shared
   pi mutexes.
3. Removal of the userspace struct pthread_mutex m_owner field.
4. The sysctl kern.ipc.umtx_vnode_persistent is added, which controls
   the lifetime of the shared mutex associated with a vnode' page.

Reviewed by:	jilles (previous version, supposedly the objection was fixed)
Discussed with:	brooks, Martin Simmons <martin@lispworks.com> (some aspects)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
2016-05-17 09:56:22 +00:00
Konstantin Belousov
2cfddaa6ff Fix umtx lock/trylock for compat32.
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
2016-04-19 11:37:43 +00:00
Konstantin Belousov
1bdbd70599 Implement process-shared locks support for libthr.so.3, without
breaking the ABI.  Special value is stored in the lock pointer to
indicate shared lock, and offline page in the shared memory is
allocated to store the actual lock.

Reviewed by:	vangyzen (previous version)
Discussed with:	deischen, emaste, jhb, rwatson,
	Martin Simmons <martin@lispworks.com>
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
2016-02-28 17:52:33 +00:00
Konstantin Belousov
50657fd342 Minor (and incomplete) style cleanup.
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2015-10-30 20:47:42 +00:00
Konstantin Belousov
2936e0013c Also mark compat32 umtx op table as constant.
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2015-10-30 19:32:30 +00:00
Konstantin Belousov
c539e87014 Use C99 array initialization, which also makes the code
self-documented, and eases addition of new ops.

For the similar reasons, eliminate UMTX_OP_MAX.  nitems() handles the
only use of the symbol.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
2015-10-30 19:20:40 +00:00
Ed Schouten
dc4b532479 Fix bad arithmetic in umtx_key_get() to compute object offset.
It looks like umtx_key_get() has the addition and subtraction the wrong
way around, meaning that it fails to match in certain cases. This causes
the cloudlibc unit tests to deadlock in certain cases.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D3287
2015-08-04 06:01:13 +00:00
Ed Schouten
52942c1eae Add missing const keyword to function parameter.
The umtx_key_get() function does not dereference the address off the
userspace object. The pointer can safely be const.
2015-08-03 21:11:33 +00:00
Eric van Gyzen
e858b027db Clean up some cosmetic nits in kern_umtx.c, found during recent work
in this area and by the Clang static analyzer.

Remove some dead assignments.

Fix a typo in a panic string.

Use umtx_pi_disown() instead of duplicate code.

Use an existing variable instead of curthread.

Approved by:	kib (mentor)
MFC after:	3 days
Sponsored by:	Dell Inc
2015-03-28 21:21:40 +00:00
Konstantin Belousov
13dad10871 The umtx_lock mutex is used by top-half of the kernel, but is
currently a spin lock.  Apparently, the only reason for this is that
umtx_thread_exit() is called under the process spinlock, which put the
requirement on the umtx_lock.  Note that the witness static order list
is wrong for the umtx_lock, umtx_lock is explicitely before any thread
lock, so it is also before sleepq locks.

Change umtx_lock to be the sleepable mutex.  For the reason above, the
calls to umtx_thread_exit() are moved from thread_exit() earlier in
each caller, when the process spin lock is not yet taken.

Discussed with:	jhb
Tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks
2015-02-28 04:19:02 +00:00
Konstantin Belousov
84b736b268 When failing to claim ownership of a umtx_pi, restore the umutex owner
to its previous, unowned state.  This avoids compounding an existing
problem of inconsistent ownership.

Submitted by:	Eric van Gyzen <eric_van_gyzen@dell.com>
Obtained from:	Dell Inc.
PR:	198914
MFC after:	1 week
2015-02-25 16:17:16 +00:00
Konstantin Belousov
cc876d2c5c When unlocking a contested PI pthread mutex, if the queue of waiters
is empty, look up the umtx_pi and disown it if the current thread owns it.
This can happen if a signal or timeout removed the last waiter from
the queue, but there is still a thread in do_lock_pi() holding a reference
on the umtx_pi.  The unlocking thread might not own the umtx_pi in this case,
but if it does, it must disown it to keep the ownership consistent between
the umtx_pi and the umutex.

Submitted by:	Eric van Gyzen <eric_van_gyzen@dell.com>
	with advice from: Elliott Rabe and Jim Muchow, also at Dell Inc.
Obtained from:	Dell Inc.
PR:	198914
2015-02-25 16:12:56 +00:00
Konstantin Belousov
6690381ef1 The dependency chain for priority-inheritance mutexes could be
subverted by userspace into cycle.  Both umtx_propagate_priority() and
umtx_repropagate_priority() would then loop infinitely, owning the
spinlock.

Check for the cycle using standard Floyd' algorithm before doing the
pass in the affected functions.  Add simple check for condition of
tricking the thread into a wait for itself, which could be easily
simulated by usermode without race.

Found by:	Eric van Gyzen <eric@vangyzen.net>
In collaboration with:	Eric van Gyzen <eric@vangyzen.net>
Tested by:	pho
MFC after:	1 week
2015-01-31 12:27:40 +00:00
Konstantin Belousov
416be7a1c6 Fix assertion, &uc->uc_busy is never zero, the intent is to test the
uc_busy value, and not its address [1].

Remove the single use of the macro, write KASSERT() explicitely in the
code of umtxq_sleep_pi().

Submitted by:	Eric van Gyzen <eric@vangyzen.net> [1]
MFC after:	1 week
2014-11-13 18:51:09 +00:00
Konstantin Belousov
2361c6d135 Add type qualifier volatile to the base (userspace) address argument
of fuword(9) and suword(9).  This makes the functions type-compatible
with volatile objects and does not require devolatile force, e.g. in
kern_umtx.c.

Requested by:	bde
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks
2014-10-31 17:43:21 +00:00
Konstantin Belousov
f7e91c288a Convert kern_umtx.c to use fueword() and casueword().
Also fix some mishandling of suword(9) errors as errno, which resulted
in spurious ERESTART.

Sponsored by:	The FreeBSD Foundation
Tested by:	pho
MFC after:	3 weeks
2014-10-28 15:30:33 +00:00
John Baldwin
1bc9ea1caa Use correct type in __DEVOLATILE(). 2014-10-25 20:42:47 +00:00
Xin LI
6362e06b42 Fix build. 2014-10-25 00:16:36 +00:00
John Baldwin
53e1ffbbce The current POSIX semaphore implementation stores the _has_waiters flag
in a separate word from the _count.  This does not permit both items to
be updated atomically in a portable manner.  As a result, sem_post()
must always perform a system call to safely clear _has_waiters.

This change removes the _has_waiters field and instead uses the high bit
of _count as the _has_waiters flag.  A new umtx object type (_usem2) and
two new umtx operations are added (SEM_WAIT2 and SEM_WAKE2) to implement
these semantics.  The older operations are still supported under the
COMPAT_FREEBSD9/10 options.  The POSIX semaphore API in libc has
been updated to use the new implementation.  Note that the new
implementation is not compatible with the previous implementation.
However, this only affects static binaries (which cannot be helped by
symbol versioning).  Binaries using a dynamic libc will continue to work
fine.  SEM_MAGIC has been bumped so that mismatched binaries will error
rather than corrupting a shared semaphore.  In addition, a padding field
has been added to sem_t so that it remains the same size.

Differential Revision:	https://reviews.freebsd.org/D961
Reported by:	adrian
Reviewed by:	kib, jilles (earlier version)
Sponsored by:	Norse
2014-10-24 20:02:44 +00:00
Konstantin Belousov
fbb6eca60f In do_lock_pi(), do not override error from umtxq_sleep_pi() when
doing suspend check.  This restores the pre-r251684 behaviour, to
retry once after the signal is detected.

PR:	kern/192918
Submitted by:	Elliott Rabe, Dell Inc., Eric van Gyzen <eric@vangyzen.net>
Obtained from:	Dell Inc.
MFC after:	1 week
2014-08-22 18:42:14 +00:00
Attilio Rao
3198603edd Fix comments.
Sponsored by:	EMC / Isilon Storage Division
2014-03-19 12:45:40 +00:00
Attilio Rao
ce42e79310 Remove dead code from umtx support:
- Retire long time unused (basically always unused) sys__umtx_lock()
  and sys__umtx_unlock() syscalls
- struct umtx and their supporting definitions
- UMUTEX_ERROR_CHECK flag
- Retire UMTX_OP_LOCK/UMTX_OP_UNLOCK from _umtx_op() syscall

__FreeBSD_version is not bumped yet because it is expected that further
breakages to the umtx interface will follow up in the next days.
However there will be a final bump when necessary.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	jhb
2014-03-18 21:32:03 +00:00
Konstantin Belousov
1d7466bca4 Fix two issues with the spin loops in the umtx(2) implementation.
- When looping, check for the pending suspension.  Otherwise, other
  usermode thread which races with the looping one, could try to
  prevent the process from stopping or exiting.

- Add missed checks for the faults from casuword*().  The code is
  structured in a way which makes the loops exit if the specified
  address is invalid, since both fuword() and casuword() return -1 on
  the fault.  But if the address is mapped readonly, the typical value
  read by fuword() is different from -1, while casuword() returns -1.
  Absent the checks for casuword() faults, this is interpreted as the
  race with other thread and causes non-interruptible spinning in the
  kernel.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
2013-06-13 09:33:22 +00:00
Jilles Tjoelker
1e367efa8b sem: Restart the POSIX sem_* calls after signals with SA_RESTART set.
Programs often do not expect an [EINTR] return from sem_wait() and POSIX
only allows it if the signal was installed without SA_RESTART. The timeout
in sem_timedwait() is absolute so it can be restarted normally.

The umtx call can be invoked with a relative timeout and in that case
[ERESTART] must be changed to [EINTR]. However, libc does not do this.

The old POSIX semaphore implementation did this correctly (before r249566),
unlike the new umtx one.

It may be desirable to avoid [EINTR] completely, which matches the pthread
functions and is explicitly permitted by POSIX. However, the kernel must
return [EINTR] at least for signals with SA_RESTART clear, otherwise pthread
cancellation will not abort a semaphore wait. In this commit, only restore
the 8.x behaviour which is also permitted by POSIX.

Discussed with:	jhb
MFC after:	1 week
2013-04-19 10:16:00 +00:00