The code is pretty much guaranteed not to be able to unlock.
This is a minor nit. The code still performs way too many reads.
The altered exclusive-locked condition is supposed to be always
true as well, to be cleaned up at a later date.
r313683 introduced new lockmgr APIs that missed the panic-time neutering
present in the rest of our locks. Correct that by adding the usual check.
Additionally, move the __lockmgr_args neutering above the assertions at the
top of the function. Drop the interlock unlock because we shouldn't have
an unneutered interlock either. No point trying to unlock it.
PR: 227749
Reported by: jtl
Sponsored by: Dell EMC Isilon
The main routine takes 8 args, 3 of which are almost the same for most uses.
This in particular pushes it above the limit of 6 arguments passable through
registers on amd64 making it impossible to tail call.
This is a prerequisite for further cleanups.
Tested by: pho
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.
The main lockmgr routine takes 8 arguments which makes it impossible to
tail-call it by the intermediate vop_stdlock/unlock routines.
The routine itself starts with an if-forest and reads from the lock itself
several times.
This slows things down both single- and multi-threaded. With the patch
single-threaded fstats go 4% up and multithreaded up to ~27%.
Note that there is still a lot of room for improvement.
Reviewed by: kib
Tested by: pho
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
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
FreeBSD developers need more time to review patches in the surrounding
areas like the TCP stack which are using MPSAFE callouts to restore
distribution of callouts on multiple CPUs.
Bump the __FreeBSD_version instead of reverting it.
Suggested by: kmacy, adrian, glebius and kib
Differential Revision: https://reviews.freebsd.org/D1438
- Close a migration race where callout_reset() failed to set the
CALLOUT_ACTIVE flag.
- Callout callback functions are now allowed to be protected by
spinlocks.
- Switching the callout CPU number cannot always be done on a
per-callout basis. See the updated timeout(9) manual page for more
information.
- The timeout(9) manual page has been updated to reflect how all the
functions inside the callout API are working. The manual page has
been made function oriented to make it easier to deduce how each of
the functions making up the callout API are working without having
to first read the whole manual page. Group all functions into a
handful of sections which should give a quick top-level overview
when the different functions should be used.
- The CALLOUT_SHAREDLOCK flag and its functionality has been removed
to reduce the complexity in the callout code and to avoid problems
about atomically stopping callouts via callout_stop(). If someone
needs it, it can be re-added. From my quick grep there are no
CALLOUT_SHAREDLOCK clients in the kernel.
- A new callout API function named "callout_drain_async()" has been
added. See the updated timeout(9) manual page for a complete
description.
- Update the callout clients in the "kern/" folder to use the callout
API properly, like cv_timedwait(). Previously there was some custom
sleepqueue code in the callout subsystem, which has been removed,
because we now allow callouts to be protected by spinlocks. This
allows us to tear down the callout like done with regular mutexes,
and a "td_slpmutex" has been added to "struct thread" to atomically
teardown the "td_slpcallout". Further the "TDF_TIMOFAIL" and
"SWT_SLEEPQTIMO" states can now be completely removed. Currently
they are marked as available and will be cleaned up in a follow up
commit.
- Bump the __FreeBSD_version to indicate kernel modules need
recompilation.
- There has been several reports that this patch "seems to squash a
serious bug leading to a callout timeout and panic".
Kernel build testing: all architectures were built
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D1438
Sponsored by: Mellanox Technologies
Reviewed by: jhb, adrian, sbruno and emaste
whether the shared request for already shared-locked lock could be
granted. Both problems result in the exclusive locker starvation.
The concurrent exclusive request is indicated by either
LK_EXCLUSIVE_WAITERS or LK_EXCLUSIVE_SPINNERS flags. The reverse
condition, i.e. no exclusive waiters, must check that both flags are
cleared.
Add a flag LK_NODDLKTREAT for shared lock request to indicate that
current thread guarantees that it does not own the lock in shared
mode. This turns back the exclusive lock starvation avoidance code;
see man page update for detailed description.
Use LK_NODDLKTREAT when doing lookup(9).
Reported and tested by: pho
No objections from: attilio
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
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
atomically upgrade shared lock to exclusive. On failure, error is
returned and lock is not dropped in the process.
Tested by: pho (previous version)
No objections from: attilio
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Approved by: re (glebius)
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)
- 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.
of a lock within a single thread.
- Fix handling of interlocks in WITNESS by properly requiring the interlock
to be held exactly once if it is specified.
locks. To support this, VNODE locks are created with the LK_IS_VNODE
flag. This flag is propagated down using the LO_IS_VNODE flag.
Note that WITNESS still records the LOR. Only the printing and the
optional entering into the kernel debugger is bypassed with the
WITNESS_NO_VNODE option.
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
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
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
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)
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
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.
PMC/SYSV/...).
No FreeBSD version bump, the userland application to query the features will
be committed last and can serve as an indication of the availablility if
needed.
Sponsored by: Google Summer of Code 2010
Submitted by: kibab
Reviewed by: arch@ (parts by rwatson, trasz, jhb)
X-MFC after: to be determined in last commit with code from this project
LK_CANRECURSE after a lock is created. Use them to implement macros that
otherwise manipulated the flags directly. Assert that the associated
lockmgr lock is exclusively locked by the current thread when manipulating
these flags to ensure the flag updates are safe. This last change required
some minor shuffling in a few filesystems to exclusively lock a brand new
vnode slightly earlier.
Reviewed by: kib
MFC after: 3 days
sleeps/timeout may have left spourious lk_exslpfail counts on, so clean
it up even when accessing a shared queue acquisition, giving to
lk_exslpfail the value of 'upper limit'.
In the worst case scenario, infact (mixed
interruptible sleep / LK_SLEEPFAIL waiters) what may happen is that both
queues are awaken even if that's not necessary, but still no harm.
Reported by: Lucius Windschuh <lwindschuh at googlemail dot com>
Reviewed by: kib
Tested by: pho, Lucius Windschuh <lwindschuh at googlemail dot com>
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
In the lockmgr support:
- GIANT_RESTORE() is just called when the sleep finishes, so the current
code can ends up into a giant unlock problem. Fix it by appropriately
call GIANT_RESTORE() when needed. Note that this is not exactly ideal
because for any interation of the adaptive spinning we drop and restore
Giant, but the overhead should be not a factor.
- In the lock held in exclusive mode case, after the adaptive spinning is
brought to completition, we should just retry to acquire the lock
instead to fallthrough. Fix that.
- Fix a style nit
In the sx support:
- Call GIANT_SAVE() before than looping. This saves some overhead because
in the current code GIANT_SAVE() is called several times.
Tested by: Giovanni Trematerra <giovanni dot trematerra at gmail dot com>
a pointer-fetching specific operation check. Consequently, rename the
operation ASSERT_ATOMIC_LOAD_PTR().
* Fix the implementation of ASSERT_ATOMIC_LOAD_PTR() by checking
directly alignment on the word boundry, for all the given specific
architectures. That's a bit too strict for some common case, but it
assures safety.
* Add a comment explaining the scope of the macro
* Add a new stub in the lockmgr specific implementation
Tested by: marcel (initial version), marius
Reviewed by: rwatson, jhb (comment specific review)
Approved by: re (kib)
Actually, as it did receive few tuning, the support is disabled by
default, but it can opt-in with the option ADAPTIVE_LOCKMGRS.
Due to the nature of lockmgrs, adaptive spinning needs to be
selectively enabled for any interested lockmgr.
The support is bi-directional, or, in other ways, it will work in both
cases if the lock is held in read or write way. In particular, the
read path is passible of further tunning using the sysctls
debug.lockmgr.retries and debug.lockmgr.loops . Ideally, such sysctls
should be axed or compiled out before release.
Addictionally note that adaptive spinning doesn't cope well with
LK_SLEEPFAIL. The reason is that many (and probabilly all) consumers
of LK_SLEEPFAIL are mainly interested in knowing if the interlock was
dropped or not in order to reacquire it and re-test initial conditions.
This directly interacts with adaptive spinning because lockmgr needs
to drop the interlock while spinning in order to avoid a deadlock
(further details in the comments inside the patch).
Final note: finding someone willing to help on tuning this with
relevant workloads would be either very important and appreciated.
Tested by: jeff, pho
Requested by: many
