linux-stable.git/ipc/sem.c, branch v4.10.2

ipc/sem.c: fix incorrect sem_lock pairing

2017-01-11T02:31:55+00:00

Based on the syzcaller test case from dvyukov:

  https://gist.githubusercontent.com/dvyukov/d0e5efefe4d7d6daed829f5c3ca26a40/raw/08d0a261fe3c987bed04fbf267e08ba04bd533ea/gistfile1.txt

The slow (i.e.: failure to acquire) syscall exit from semtimedop()
incorrectly assumed that the the same lock is acquired as it was at the
initial syscall entry.

This is wrong:
 - thread A: single semop semop(), sleeps
 - thread B: multi semop semop(), sleeps
 - thread A: woken up by signal/timeout

With this sequence, the initial sem_lock() call locks the per-semaphore
spinlock, and it is unlocked with sem_unlock().  The call at the syscall
return locks the global spinlock.  Because locknum is not updated, the
following sem_unlock() call unlocks the per-semaphore spinlock, which is
actually not locked.

The fix is trivial: Use the return value from sem_lock.

Fixes: 370b262c896e ("ipc/sem: avoid idr tree lookup for interrupted semop")
Link: http://lkml.kernel.org/r/1482215645-22328-1-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul 
Reported-by: Dmitry Vyukov 
Reported-by: Johanna Abrahamsson 
Tested-by: Johanna Abrahamsson 
Acked-by: Davidlohr Bueso 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: avoid idr tree lookup for interrupted semop

2016-12-15T00:04:08+00:00

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 
Cc: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: simplify wait-wake loop

2016-12-15T00:04:08+00:00

Instead of using the reverse goto, we can simplify the flow and make it
more language natural by just doing do-while instead.  One would hope
this is the standard way (or obviously just with a while bucle) that we
do wait/wakeup handling in the kernel.  The exact same logic is kept,
just more indented.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/1478708774-28826-2-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso 
Cc: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: use proper list api for pending_list wakeups

2016-12-15T00:04:08+00:00

... saves some LoC and looks cleaner than re-implementing the calls.

Link: http://lkml.kernel.org/r/1474225896-10066-6-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso 
Acked-by: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: explicitly inline check_restart

2016-12-15T00:04:08+00:00

The compiler already does this, but make it explicit.  This helper is
really small and also used in update_queue's main loop, which is O(N^2)
scanning.  Inline and avoid the function overhead.

Link: http://lkml.kernel.org/r/1474225896-10066-5-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso 
Cc: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: optimize perform_atomic_semop()

2016-12-15T00:04:08+00:00

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 
Cc: Manfred Spraul 
Signed-off-by: Colin Ian King 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: rework task wakeups

2016-12-15T00:04:08+00:00

Our sysv sems have been using the notion of lockless wakeups for a
while, ever since commit 0a2b9d4c7967 ("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 
Acked-by: Manfred Spraul 
Signed-off-by: Stephen Rothwell 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem: do not call wake_sem_queue_do() prematurely ... as this call should obviously be paired with its _prepare()

2016-12-15T00:04:08+00:00

counterpart.  At least whenever possible, as there is no harm in calling
it bogusly as we do now in a few places.  Immediate error semop(2) paths
that are far from ever having the task block can be simplified and avoid
a few unnecessary loads on their way out of the call as it is not deeply
nested.

Link: http://lkml.kernel.org/r/1474225896-10066-2-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso 
Cc: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem.c: add cond_resched in exit_sme

2016-10-11T22:06:33+00:00

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:[]  [] _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 
Cc: Herton R. Krzesinski 
Cc: Fabian Frederick 
Cc: Davidlohr Bueso 
Cc: Manfred Spraul 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

ipc/sem.c: fix complex_count vs. simple op race

2016-10-11T22:06:33+00:00

Commit 6d07b68ce16a ("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 6d07b68ce16a ("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: 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()")
Link: http://lkml.kernel.org/r/1469123695-5661-1-git-send-email-manfred@colorfullife.com
Reported-by: 
Cc: "H. Peter Anvin" 
Cc: Peter Zijlstra 
Cc: Davidlohr Bueso 
Cc: Thomas Gleixner 
Cc: Ingo Molnar 
Cc: <1vier1@web.de>
Cc: 	[3.10+]
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds