We are going to split <linux/sched/wake_q.h> out of <linux/sched.h>, which
will have to be picked up from other headers and a couple of .c files.
Create a trivial placeholder <linux/sched/wake_q.h> file that just
maps to <linux/sched.h> to make this patch obviously correct and
bisectable.
Include the new header in the files that are going to need it.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
sysv sem has two lock modes: One with per-semaphore locks, one lock mode
with a single global lock for the whole array. When switching from the
per-semaphore locks to the global lock, all per-semaphore locks must be
scanned for ongoing operations.
The patch adds a hysteresis for switching from the global lock to the
per semaphore locks. This reduces how often the per-semaphore locks
must be scanned.
Compared to the initial patch, this is a simplified solution: Setting
USE_GLOBAL_LOCK_HYSTERESIS to 1 restores the current behavior.
In theory, a workload with exactly 10 simple sops and then one complex
op now scales a bit worse, but this is pure theory: If there is
concurrency, the it won't be exactly 10:1:10:1:10:1:... If there is no
concurrency, then there is no need for scalability.
Link: http://lkml.kernel.org/r/1476851896-3590-3-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: <1vier1@web.de>
Cc: kernel test robot <xiaolong.ye@intel.com>
Cc: <felixh@informatik.uni-bremen.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
We can avoid the idr tree lookup (albeit possibly avoiding
idr_find_fast()) when being awoken in EINTR, as the semid will not
change in this context while blocked. Use the sma pointer directly and
take the sem_lock, then re-check for RMID races. We continue to
re-check the queue.status with the lock held such that we can detect
situations where we where are dealing with a spurious wakeup but another
task that holds the sem_lock updated the queue.status while we were
spinning for it. Once we take the lock it obviously won't change again.
Being the only caller, get rid of sem_obtain_lock() altogether.
Link: http://lkml.kernel.org/r/1478708774-28826-3-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This is the main workhorse that deals with semop user calls such that
the waitforzero or semval update operations, on the set, can complete on
not as the sma currently stands. Currently, the set is iterated twice
(setting semval, then backwards for the sempid value). Slowpaths, and
particularly SEM_UNDO calls, must undo any altered sem when it is
detected that the caller must block or has errored-out.
With larger sets, there can occur situations where this involves a lot
of cycles and can obviously be a suboptimal use of cached resources in
shared memory. Ie, discarding CPU caches that are also calling semop
and have the sembuf cached (and can complete), while the current lock
holder doing the semop will block, error, or does a waitforzero
operation.
This patch proposes still iterating the set twice, but the first scan is
read-only, and we perform the actual updates afterward, once we know
that the call will succeed. In order to not suffer from the overhead of
dealing with sops that act on the same sem_num, such (rare) cases use
perform_atomic_semop_slow(), which is exactly what we have now.
Duplicates are detected before grabbing sem_lock, and uses simple a
32/64-bit hash array variable to based on the sem_num we are working on.
In addition add some comments to when we expect to the caller to block.
[akpm@linux-foundation.org: coding-style fixes]
[colin.king@canonical.com: ensure we left shift a ULL rather than a 32 bit integer]
Link: http://lkml.kernel.org/r/20161028181129.7311-1-colin.king@canonical.com
Link: http://lkml.kernel.org/r/20160921194603.GB21438@linux-80c1.suse
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Our sysv sems have been using the notion of lockless wakeups for a
while, ever since commit 0a2b9d4c79 ("ipc/sem.c: move wake_up_process
out of the spinlock section"), in order to reduce the sem_lock hold
times. This in-house pending queue can be replaced by wake_q (just like
all the rest of ipc now), in that it provides the following advantages:
o Simplifies and gets rid of unnecessary code.
o We get rid of the IN_WAKEUP complexities. Given that wake_q_add()
grabs reference to the task, if awoken due to an unrelated event,
between the wake_q_add() and wake_up_q() window, we cannot race with
sys_exit and the imminent call to wake_up_process().
o By not spinning IN_WAKEUP, we no longer need to disable preemption.
In consequence, the wakeup paths (after schedule(), that is) must
acknowledge an external signal/event, as well spurious wakeup occurring
during the pending wakeup window. Obviously no changes in semantics
that could be visible to the user. The fastpath is _only_ for when we
know for sure that we were awoken due to a the waker's successful semop
call (queue.status is not -EINTR).
On a 48-core Haswell, running the ipcscale 'waitforzero' test, the
following is seen with increasing thread counts:
v4.8-rc5 v4.8-rc5
semopv2
Hmean sembench-sem-2 574733.00 ( 0.00%) 578322.00 ( 0.62%)
Hmean sembench-sem-8 811708.00 ( 0.00%) 824689.00 ( 1.59%)
Hmean sembench-sem-12 842448.00 ( 0.00%) 845409.00 ( 0.35%)
Hmean sembench-sem-21 933003.00 ( 0.00%) 977748.00 ( 4.80%)
Hmean sembench-sem-48 935910.00 ( 0.00%) 1004759.00 ( 7.36%)
Hmean sembench-sem-79 937186.00 ( 0.00%) 983976.00 ( 4.99%)
Hmean sembench-sem-234 974256.00 ( 0.00%) 1060294.00 ( 8.83%)
Hmean sembench-sem-265 975468.00 ( 0.00%) 1016243.00 ( 4.18%)
Hmean sembench-sem-296 991280.00 ( 0.00%) 1042659.00 ( 5.18%)
Hmean sembench-sem-327 975415.00 ( 0.00%) 1029977.00 ( 5.59%)
Hmean sembench-sem-358 1014286.00 ( 0.00%) 1049624.00 ( 3.48%)
Hmean sembench-sem-389 972939.00 ( 0.00%) 1043127.00 ( 7.21%)
Hmean sembench-sem-420 981909.00 ( 0.00%) 1056747.00 ( 7.62%)
Hmean sembench-sem-451 990139.00 ( 0.00%) 1051609.00 ( 6.21%)
Hmean sembench-sem-482 965735.00 ( 0.00%) 1040313.00 ( 7.72%)
[akpm@linux-foundation.org: coding-style fixes]
[sfr@canb.auug.org.au: merge fix for WAKE_Q to DEFINE_WAKE_Q rename]
Link: http://lkml.kernel.org/r/20161122210410.5eca9fc2@canb.auug.org.au
Link: http://lkml.kernel.org/r/1474225896-10066-3-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
In CONFIG_PREEMPT=n kernel a softlockup was observed while the for loop in
exit_sem. Apparently it's possible for the loop to take quite a long time
and it doesn't have a scheduling point in it. Since the codes is
executing under an rcu read section this may also cause rcu stalls, which
in turn block synchronize_rcu operations, which more or less de-stabilises
the whole system.
Fix this by introducing a cond_resched() at the beginning of the loop.
So this patch fixes the following:
NMI watchdog: BUG: soft lockup - CPU#10 stuck for 23s! [httpd:18119]
CPU: 10 PID: 18119 Comm: httpd Tainted: G O 4.4.20-clouder2 #6
Hardware name: Supermicro X10DRi/X10DRi, BIOS 1.1 04/14/2015
task: ffff88348d695280 ti: ffff881c95550000 task.ti: ffff881c95550000
RIP: 0010:[<ffffffff81614bc7>] [<ffffffff81614bc7>] _raw_spin_lock+0x17/0x30
RSP: 0018:ffff881c95553e40 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff883161b1eea8 RCX: 000000000000000d
RDX: 0000000000000001 RSI: 000000000000000e RDI: ffff883161b1eea4
RBP: ffff881c95553ea0 R08: ffff881c95553e68 R09: ffff883fef376f88
R10: ffff881fffb58c20 R11: ffffea0072556600 R12: ffff883161b1eea0
R13: ffff88348d695280 R14: ffff883dec427000 R15: ffff8831621672a0
FS: 0000000000000000(0000) GS:ffff881fffb40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3b3723e020 CR3: 0000000001c0a000 CR4: 00000000001406e0
Call Trace:
? exit_sem+0x7c/0x280
do_exit+0x338/0xb40
do_group_exit+0x43/0xd0
SyS_exit_group+0x14/0x20
entry_SYSCALL_64_fastpath+0x16/0x6e
Link: http://lkml.kernel.org/r/1475154992-6363-1-git-send-email-kernel@kyup.com
Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Cc: Herton R. Krzesinski <herton@redhat.com>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Commit 6d07b68ce1 ("ipc/sem.c: optimize sem_lock()") introduced a
race:
sem_lock has a fast path that allows parallel simple operations.
There are two reasons why a simple operation cannot run in parallel:
- a non-simple operations is ongoing (sma->sem_perm.lock held)
- a complex operation is sleeping (sma->complex_count != 0)
As both facts are stored independently, a thread can bypass the current
checks by sleeping in the right positions. See below for more details
(or kernel bugzilla 105651).
The patch fixes that by creating one variable (complex_mode)
that tracks both reasons why parallel operations are not possible.
The patch also updates stale documentation regarding the locking.
With regards to stable kernels:
The patch is required for all kernels that include the
commit 6d07b68ce1 ("ipc/sem.c: optimize sem_lock()") (3.10?)
The alternative is to revert the patch that introduced the race.
The patch is safe for backporting, i.e. it makes no assumptions
about memory barriers in spin_unlock_wait().
Background:
Here is the race of the current implementation:
Thread A: (simple op)
- does the first "sma->complex_count == 0" test
Thread B: (complex op)
- does sem_lock(): This includes an array scan. But the scan can't
find Thread A, because Thread A does not own sem->lock yet.
- the thread does the operation, increases complex_count,
drops sem_lock, sleeps
Thread A:
- spin_lock(&sem->lock), spin_is_locked(sma->sem_perm.lock)
- sleeps before the complex_count test
Thread C: (complex op)
- does sem_lock (no array scan, complex_count==1)
- wakes up Thread B.
- decrements complex_count
Thread A:
- does the complex_count test
Bug:
Now both thread A and thread C operate on the same array, without
any synchronization.
Fixes: 6d07b68ce1 ("ipc/sem.c: optimize sem_lock()")
Link: http://lkml.kernel.org/r/1469123695-5661-1-git-send-email-manfred@colorfullife.com
Reported-by: <felixh@informatik.uni-bremen.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: <1vier1@web.de>
Cc: <stable@vger.kernel.org> [3.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
As indicated by bug#112271, Linux sets the sempid value upon semctl, and
not only for semop calls. However, within semctl we only do this for
SETVAL, leaving SETALL without updating the field, and therefore rather
inconsistent behavior when compared to other Unices.
There is really no documentation regarding this and therefore users
should not make assumptions. With this patch, along with updating
semctl.2 manpages, this scenario should become less ambiguous As such,
set sempid on SETALL cmd.
Also update some in-code documentation, specifying where the sempid is
set.
Passes ltp and custom testcase where a child (fork) does SETALL to the
set.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Philip Semanchuk <linux_kernel.20.ick@spamgourmet.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Herton R. Krzesinski <herton@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Kees Cook
Manfred Spraul
Ingo Molnar
Davidlohr Bueso
Nikolay Borisov
Fabian Frederick
Peter Zijlstra