Commit Graph

120 Commits

Author SHA1 Message Date
Mateusz Guzik
a21018063b locks: ensure proper barriers are used with atomic ops when necessary
Unclear how, but the locking routine for mutexes was using the *release*
barrier instead of acquire. This must have been either a copy-pasto or bad
completion.

Going through other uses of atomics shows no barriers in:
- upgrade routines (addressed in this patch)
- sections protected with turnstile locks - this should be fine as necessary
  barriers are in the worst case provided by turnstile unlock

I would like to thank Mark Millard and andreast@ for reporting the problem and
testing previous patches before the issue got identified.

ps.
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|             \
|              \
\           _  \
,\  _    ,'-,/-)\
( * \ \,' ,' ,'-)
 `._,)     -',-')
   \/         ''/
    )        / /
   /       ,'-'

Hardware provided by: IBM LTC
2017-03-01 05:06:21 +00:00
Mateusz Guzik
b247fd395d locks: make trylock routines check for 'unowned' value
Since fcmpset can fail without lock contention e.g. on arm, it was possible
to get spurious failures when the caller was expecting the primitive to succeed.

Reported by:	mmel
2017-02-19 16:28:46 +00:00
Mateusz Guzik
5c5df0d99b locks: clean up trylock primitives
In particular thius reduces accesses of the lock itself.
2017-02-18 22:06:03 +00:00
Mateusz Guzik
0108a98012 sx: fix compilation on UP kernels after r313855
sx primitives use inlines as opposed to macros. Change the tested condition
to LOCK_DEBUG which covers the case, but is slightly overzelaous.

Reported by:	kib
2017-02-17 10:58:12 +00:00
Mateusz Guzik
ffd5c94c4f locks: let primitives for modules unlock without always goging to the slsow path
It is only needed if the LOCK_PROFILING is enabled. It has to always check if
the lock is about to be released which requires an avoidable read if the option
is not specified..
2017-02-17 05:39:40 +00:00
Mateusz Guzik
afa39f7a32 locks: remove SCHEDULER_STOPPED checks from primitives for modules
They all fallback to the slow path if necessary and the check is there.

This means a panicked kernel executing code from modules will be able to
succeed doing actual lock/unlock, but this was already the case for core code
which has said primitives inlined.
2017-02-17 05:09:51 +00:00
Mateusz Guzik
3b3cf014fc locks: tidy up unlock fallback paths
Update comments to note these functions are reachable if lockstat is
enabled.

Check if the lock has any bits set before attempting unlock, which saves
an unnecessary atomic operation.
2017-02-09 08:19:30 +00:00
Mateusz Guzik
834f70f32f sx: implement slock/sunlock fast path
See r313454.
2017-02-08 19:29:34 +00:00
Mateusz Guzik
8e5a3e9a9d locks: change backoff to exponential
Previous implementation would use a random factor to spread readers and
reduce chances of starvation. This visibly reduces effectiveness of the
mechanism.

Switch to the more traditional exponential variant. Try to limit starvation
by imposing an upper limit of spins after which spinning is half of what
other threads get. Note the mechanism is turned off by default.

Reviewed by:	kib (previous version)
2017-02-07 14:49:36 +00:00
Mateusz Guzik
c1aaf63cb5 locks: fix recursion support after recent changes
When a relevant lockstat probe is enabled the fallback primitive is called with
a constant signifying a free lock. This works fine for typical cases but breaks
with recursion, since it checks if the passed value is that of the executing
thread.

Read the value if necessary.
2017-02-06 09:40:14 +00:00
Mateusz Guzik
6ebb77b6a6 sx: move lockstat handling out of inline primitives
See r313275 for details.
2017-02-05 09:54:16 +00:00
Mateusz Guzik
3ae56ce958 sx: add witness support missed in r313272 2017-02-05 06:51:45 +00:00
Mateusz Guzik
9d2e4290ff sx: uninline slock/sunlock
Shared locking routines explicitly read the value and test it. If the
change attempt fails, they fall back to a regular function which would
retry in a loop.

The problem is that with many concurrent readers the risk of failure is pretty
high and even the value returned by fcmpset is very likely going to be stale
by the time the loop in the fallback routine is reached.

Uninline said primitives. It gives a throughput increase when doing concurrent
slocks/sunlocks with 80 hardware threads from ~50 mln/s to ~56 mln/s.

Interestingly, rwlock primitives are already not inlined.
2017-02-05 05:20:29 +00:00
Mateusz Guzik
fa47404353 sx: switch to fcmpset
Discussed with:	jhb
Tested by:	pho (previous version)
2017-02-05 04:54:20 +00:00
Mateusz Guzik
290511163d Sprinkle __read_mostly on backoff and lock profiling code.
MFC after:	1 month
2017-01-27 15:03:51 +00:00
Mateusz Guzik
c5f61e6f96 sx: reduce lock accesses similarly to r311172
Discussed with:	jhb
Tested by:	pho (previous version)
2017-01-18 17:55:08 +00:00
Mark Johnston
c365a2934e Return a non-NULL owner only if the lock is exclusively held in owner_sx().
Fix some whitespace bugs while here.

MFC after:	2 weeks
2016-12-10 02:56:44 +00:00
Mateusz Guzik
0453ade508 locks: fix sx compilation on mips after r303643
The kernel.h header is required for the SYSINIT macro, which apparently
was present on amd64 by accident.

Reported by:	kib
2016-08-03 09:15:10 +00:00
Mateusz Guzik
fa5000a4f3 locks: fix compilation for KDTRACE_HOOKS && !ADAPTIVE_* case
Reported by:	Michael Butler <imb protected-networks.net>
2016-08-02 03:05:59 +00:00
Mateusz Guzik
0412689595 locks: fix up ifdef guards introduced in r303643
Both sx and rwlocks had copy-pasted ADAPTIVE_MUTEXES instead of the correct
define.

MFC after:	1 week
2016-08-02 00:15:08 +00:00
Mateusz Guzik
1ada904147 Implement trivial backoff for locking primitives.
All current spinning loops retry an atomic op the first chance they get,
which leads to performance degradation under load.

One classic solution to the problem consists of delaying the test to an
extent. This implementation has a trivial linear increment and a random
factor for each attempt.

For simplicity, this first thouch implementation only modifies spinning
loops where the lock owner is running. spin mutexes and thread lock were
not modified.

Current parameters are autotuned on boot based on mp_cpus.

Autotune factors are very conservative and are subject to change later.

Reviewed by:	kib, jhb
Tested by:	pho
MFC after:	1 week
2016-08-01 21:48:37 +00:00
Mateusz Guzik
61852185ba locks: change sleep_cnt and spin_cnt types to u_int
Both variables are uint64_t, but they only count spins or sleeps.
All reasonable values which we can get here comfortably hit in 32-bit range.

Suggested by: kib
MFC after:	1 week
2016-07-31 12:11:55 +00:00
Mateusz Guzik
e0c45af904 sx: increment spin_cnt before cpu_spinwait in xlock
The change is a no-op only done for consistency with the rest of the file.
2016-07-30 22:23:31 +00:00
Mateusz Guzik
fc4f686d59 Microoptimize locking primitives by avoiding unnecessary atomic ops.
Inline version of primitives do an atomic op and if it fails they fallback to
actual primitives, which immediately retry the atomic op.

The obvious optimisation is to check if the lock is free and only then proceed
to do an atomic op.

Reviewed by:	jhb, vangyzen
2016-06-01 18:32:20 +00:00
Mark Johnston
ce1c953ee0 Don't modify curthread->td_locks unless INVARIANTS is enabled.
This field is only used in a KASSERT that verifies that no locks are held
when returning to user mode. Moreover, the td_locks accounting is only
correct when LOCK_DEBUG > 0, which is implied by INVARIANTS.

Reviewed by:	jhb
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D3205
2015-08-02 00:03:08 +00:00
Mark Johnston
de2c95cc00 Consistently use a reader/writer flag for lockstat probes in rwlock(9) and
sx(9), rather than using the probe function name to determine whether a
given lock is a read lock or a write lock. Update lockstat(1) accordingly.
2015-07-19 22:24:33 +00:00
Mark Johnston
32cd0147fa Implement the lockstat provider using SDT(9) instead of the custom provider
in lockstat.ko. This means that lockstat probes now have typed arguments and
will utilize SDT probe hot-patching support when it arrives.

Reviewed by:	gnn
Differential Revision:	https://reviews.freebsd.org/D2993
2015-07-19 22:14:09 +00:00
Mark Johnston
e2b25737ee Pass the lock object to lockstat_nsecs() and return immediately if
LO_NOPROFILE is set. Some timecounter handlers acquire a spin mutex, and
we don't want to recurse if lockstat probes are enabled.

PR:		201642
Reviewed by:	avg
MFC after:	3 days
2015-07-18 00:57:30 +00:00
Andriy Gapon
076dd8eb2e several lockstat improvements
0. For spin events report time spent spinning, not a loop count.
While loop count is much easier and cheaper to obtain it is hard
to reason about the reported numbers, espcially for adaptive locks
where both spinning and sleeping can happen.
So, it's better to compare apples and apples.

1. Teach lockstat about FreeBSD rw locks.
This is done in part by changing the corresponding probes
and in part by changing what probes lockstat should expect.

2. Teach lockstat that rw locks are adaptive and can spin on FreeBSD.

3. Report lock acquisition events for successful rw try-lock operations.

4. Teach lockstat about FreeBSD sx locks.
Reporting of events for those locks completely mirrors
rw locks.

5. Report spin and block events before acquisition event.
This is behavior documented for the upstream, so it makes sense to stick
to it.  Note that because of FreeBSD adaptive lock implementations
both the spin and block events may be reported for the same acquisition
while the upstream reports only one of them.

Differential Revision:	https://reviews.freebsd.org/D2727
Reviewed by:	markj
MFC after:	17 days
Relnotes:	yes
Sponsored by:	ClusterHQ
2015-06-12 10:01:24 +00:00
Dmitry Chagin
fd07ddcf6f Add _NEW flag to mtx(9), sx(9), rmlock(9) and rwlock(9).
A _NEW flag passed to _init_flags() to avoid check for double-init.

Differential Revision:	https://reviews.freebsd.org/D1208
Reviewed by:	jhb, wblock
MFC after:	1 Month
2014-12-13 21:00:10 +00:00
John Baldwin
2cba8dd301 Add a new thread state "spinning" to schedgraph and add tracepoints at the
start and stop of spinning waits in lock primitives.
2014-11-04 16:35:56 +00:00
Attilio Rao
54366c0bd7 - For kernel compiled only with KDTRACE_HOOKS and not any lock debugging
option, unbreak the lock tracing release semantic by embedding
  calls to LOCKSTAT_PROFILE_RELEASE_LOCK() direclty in the inlined
  version of the releasing functions for mutex, rwlock and sxlock.
  Failing to do so skips the lockstat_probe_func invokation for
  unlocking.
- As part of the LOCKSTAT support is inlined in mutex operation, for
  kernel compiled without lock debugging options, potentially every
  consumer must be compiled including opt_kdtrace.h.
  Fix this by moving KDTRACE_HOOKS into opt_global.h and remove the
  dependency by opt_kdtrace.h for all files, as now only KDTRACE_FRAMES
  is linked there and it is only used as a compile-time stub [0].

[0] immediately shows some new bug as DTRACE-derived support for debug
in sfxge is broken and it was never really tested.  As it was not
including correctly opt_kdtrace.h before it was never enabled so it
was kept broken for a while.  Fix this by using a protection stub,
leaving sfxge driver authors the responsibility for fixing it
appropriately [1].

Sponsored by:	EMC / Isilon storage division
Discussed with:	rstone
[0] Reported by:	rstone
[1] Discussed with:	philip
2013-11-25 07:38:45 +00:00
Davide Italiano
cf6b879fad Consistently use the same value to indicate exclusively-held and
shared-held locks for all the primitives in lc_lock/lc_unlock routines.
This fixes the problems introduced in r255747, which indeed introduced an
inversion in the logic.

Reported by:	many
Tested by:	bdrewery, pho, lme, Adam McDougall, O. Hartmann
Approved by:	re (glebius)
2013-09-22 14:09:07 +00:00
Davide Italiano
7faf4d90e8 Fix lc_lock/lc_unlock() support for rmlocks held in shared mode. With
current lock classes KPI it was really difficult because there was no
way to pass an rmtracker object to the lock/unlock routines. In order
to accomplish the task, modify the aforementioned functions so that
they can return (or pass as argument) an uinptr_t, which is in the rm
case used to hold a pointer to struct rm_priotracker for current
thread. As an added bonus, this fixes rm_sleep() in the rm shared
case, which right now can communicate priotracker structure between
lc_unlock()/lc_lock().

Suggested by:	jhb
Reviewed by:	jhb
Approved by:	re (delphij)
2013-09-20 23:06:21 +00:00
John Baldwin
b5fb43e572 A few mostly cosmetic nits to aid in debugging:
- Call lock_init() first before setting any lock_object fields in
  lock init routines.  This way if the machine panics due to a duplicate
  init the lock's original state is preserved.
- Somewhat similarly, don't decrement td_locks and td_slocks until after
  an unlock operation has completed successfully.
2013-06-25 20:23:08 +00:00
Attilio Rao
cd2fe4e632 Fixup r240424: On entering KDB backends, the hijacked thread to run
interrupt context can still be idlethread. At that point, without the
panic condition, it can still happen that idlethread then will try to
acquire some locks to carry on some operations.

Skip the idlethread check on block/sleep lock operations when KDB is
active.

Reported by:	jh
Tested by:	jh
MFC after:	1 week
2012-12-22 09:37:34 +00:00
Attilio Rao
0a15e5d30d Remove all the checks on curthread != NULL with the exception of some MD
trap checks (eg. printtrap()).

Generally this check is not needed anymore, as there is not a legitimate
case where curthread != NULL, after pcpu 0 area has been properly
initialized.

Reviewed by:	bde, jhb
MFC after:	1 week
2012-09-13 22:26:22 +00:00
Attilio Rao
e3ae0dfe69 Improve check coverage about idle threads.
Idle threads are not allowed to acquire any lock but spinlocks.
Deny any attempt to do so by panicing at the locking operation
when INVARIANTS is on. Then, remove the check on blocking on a
turnstile.
The check in sleepqueues is left because they are not allowed to use
tsleep() either which could happen still.

Reviewed by:	bde, jhb, kib
MFC after:	1 week
2012-09-12 22:10:53 +00:00
Fabien Thomas
f5f9340b98 Add software PMC support.
New kernel events can be added at various location for sampling or counting.
This will for example allow easy system profiling whatever the processor is
with known tools like pmcstat(8).

Simultaneous usage of software PMC and hardware PMC is possible, for example
looking at the lock acquire failure, page fault while sampling on
instructions.

Sponsored by: NETASQ
MFC after:	1 month
2012-03-28 20:58:30 +00:00
Andriy Gapon
7a7ce668ef put sys/systm.h at its proper place or add it if missing
Reported by:	lstewart, tinderbox
Pointyhat to:	avg, attilio
MFC after:	1 week
MFC with:	r228430
2011-12-12 10:05:13 +00:00
Andriy Gapon
353705930f panic: add a switch and infrastructure for stopping other CPUs in SMP case
Historical behavior of letting other CPUs merily go on is a default for
time being.  The new behavior can be switched on via
kern.stop_scheduler_on_panic tunable and sysctl.

Stopping of the CPUs has (at least) the following benefits:
- more of the system state at panic time is preserved intact
- threads and interrupts do not interfere with dumping of the system
  state

Only one thread runs uninterrupted after panic if stop_scheduler_on_panic
is set.  That thread might call code that is also used in normal context
and that code might use locks to prevent concurrent execution of certain
parts.  Those locks might be held by the stopped threads and would never
be released.  To work around this issue, it was decided that instead of
explicit checks for panic context, we would rather put those checks
inside the locking primitives.

This change has substantial portions written and re-written by attilio
and kib at various times.  Other changes are heavily based on the ideas
and patches submitted by jhb and mdf.  bde has provided many insights
into the details and history of the current code.

The new behavior may cause problems for systems that use a USB keyboard
for interfacing with system console.  This is because of some unusual
locking patterns in the ukbd code which have to be used because on one
hand ukbd is below syscons, but on the other hand it has to interface
with other usb code that uses regular mutexes/Giant for its concurrency
protection.  Dumping to USB-connected disks may also be affected.

PR:			amd64/139614 (at least)
In cooperation with:	attilio, jhb, kib, mdf
Discussed with:		arch@, bde
Tested by:		Eugene Grosbein <eugen@grosbein.net>,
			gnn,
			Steven Hartland <killing@multiplay.co.uk>,
			glebius,
			Andrew Boyer <aboyer@averesystems.com>
			(various versions of the patch)
MFC after:		3 months (or never)
2011-12-11 21:02:01 +00:00
Attilio Rao
9fde98bba3 Introduce the same mutex-wise fix in r227758 for sx locks.
The functions that offer file and line specifications are:
- sx_assert_
- sx_downgrade_
- sx_slock_
- sx_slock_sig_
- sx_sunlock_
- sx_try_slock_
- sx_try_xlock_
- sx_try_upgrade_
- sx_unlock_
- sx_xlock_
- sx_xlock_sig_
- sx_xunlock_

Now vm_map locking is fully converted and can avoid to know specifics
about locking procedures.
Reviewed by:	kib
MFC after:	1 month
2011-11-21 12:59:52 +00:00
Pawel Jakub Dawidek
d576deedb5 Constify arguments for locking KPIs where possible.
This enables locking consumers to pass their own structures around as const and
be able to assert locks embedded into those structures.

Reviewed by:	ed, kib, jhb
2011-11-16 21:51:17 +00:00
Ed Schouten
6472ac3d8a Mark all SYSCTL_NODEs static that have no corresponding SYSCTL_DECLs.
The SYSCTL_NODE macro defines a list that stores all child-elements of
that node. If there's no SYSCTL_DECL macro anywhere else, there's no
reason why it shouldn't be static.
2011-11-07 15:43:11 +00:00
Jeff Roberson
e4cd31dd3c - Merge changes to the base system to support OFED. These include
a wider arg2 for sysctl, updates to vlan code, IFT_INFINIBAND,
   and other miscellaneous small features.
2011-03-21 09:40:01 +00:00
Matthew D Fleming
fbbb13f962 sysctl(9) cleanup checkpoint: amd64 GENERIC builds cleanly.
Commit the kernel changes.
2011-01-12 19:54:19 +00:00
John Baldwin
58ccf5b41c Remove unneeded includes of <sys/linker_set.h>. Other headers that use
it internally contain nested includes.

Reviewed by:	bde
2011-01-11 13:59:06 +00:00
John Baldwin
8545538b6a Fix a sign bug that caused adaptive spinning in sx_xlock() to not work
properly.  Among other things it did not drop Giant while spinning
leading to livelocks.

Reviewed by:	rookie, kib, jmallett
MFC after:	3 days
2010-06-08 16:17:47 +00:00
Attilio Rao
2028867def In current code, threads performing an interruptible sleep (on both
sxlock, via the sx_{s, x}lock_sig() interface, or plain lockmgr), will
leave the waiters flag on forcing the owner to do a wakeup even when if
the waiter queue is empty.
That operation may lead to a deadlock in the case of doing a fake wakeup
on the "preferred" (based on the wakeup algorithm) queue while the other
queue has real waiters on it, because nobody is going to wakeup the 2nd
queue waiters and they will sleep indefinitively.

A similar bug, is present, for lockmgr in the case the waiters are
sleeping with LK_SLEEPFAIL on.  In this case, even if the waiters queue
is not empty, the waiters won't progress after being awake but they will
just fail, still not taking care of the 2nd queue waiters (as instead the
lock owned doing the wakeup would expect).

In order to fix this bug in a cheap way (without adding too much locking
and complicating too much the semantic) add a sleepqueue interface which
does report the actual number of waiters on a specified queue of a
waitchannel (sleepq_sleepcnt()) and use it in order to determine if the
exclusive waiters (or shared waiters) are actually present on the lockmgr
(or sx) before to give them precedence in the wakeup algorithm.
This fix alone, however doesn't solve the LK_SLEEPFAIL bug. In order to
cope with it, add the tracking of how many exclusive LK_SLEEPFAIL waiters
a lockmgr has and if all the waiters on the exclusive waiters queue are
LK_SLEEPFAIL just wake both queues.

The sleepq_sleepcnt() introduction and ABI breakage require
__FreeBSD_version bumping.

Reported by:	avg, kib, pho
Reviewed by:	kib
Tested by:	pho
2009-12-12 21:31:07 +00:00
Attilio Rao
ddce63ca73 When releasing a read/shared lock we need to use a write memory barrier
in order to avoid, on architectures which doesn't have strong ordered
writes, CPU instructions reordering.

Diagnosed by:	fabio
Reviewed by:	jhb
Tested by:	Giovanni Trematerra
		<giovanni dot trematerra at gmail dot com>
2009-09-30 13:26:31 +00:00