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.
When waiters/writer spinner flags are set no new readers can show up unless
they already have a different rw rock read locked. The change in r326195 failed
to take that into account - in presence of new readers it would spin until
they all drain, which would be lead to trouble if e.g. they go off cpu and
can get scheduled because of this thread.
Reported by: pho
In order to go to sleep threads set waiter flags, but that can spuriously
fail e.g. when a new reader arrives. Instead of unlocking everything and
looping back, re-evaluate the new state while still holding the lock necessary
to go to sleep.
The pair is of use only in debug or LOCKPROF kernels, but was passed (zeroed)
for many locks even in production kernels.
While here whack the tid argument from wlock hard and xlock hard.
There is no kbi change of any sort - "external" primitives still accept the
pair.
Previous code would always spin once before checking the lock. But a lock
with e.g. 6 readers is not going to become free in the duration of once spin
even if they start draining immediately.
Conservatively perform one for each reader.
Note that the total number of allowed spins is still extremely small and is
subject to change later.
MFC after: 1 week
spin first instant of instantly re-readoing and don't re-read after
spinning is finished - the state is already known.
Note the code is subject to significant changes later.
MFC after: 1 week
Most of the lock slowpaths assert that the calling thread isn't an idle
thread. However, this may not be true if the system has panicked, and in
some cases the assertion appears before a SCHEDULER_STOPPED() check.
MFC after: 3 days
Sponsored by: Dell EMC Isilon
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
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
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..
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.
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.
This improves singlethreaded throughput on my test machine from ~247 mln
ops/s to ~328 mln.
It is mostly about avoiding the setup cost of lockstat.
Reviewed by: jhb (previous version)
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)
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.
See r313275 for details.
One difference here is that recursion handling was removed from the fallback
routine. As it is it was never supposed to see a recursed lock in the first
place. Future changes will move it out of inline variants, but right now
there is no easy to way to test if the lock is recursed without reading
additional words.
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
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
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
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
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
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
