Commit Graph

13 Commits

Author SHA1 Message Date
Paul E. McKenney
d372e20433 Documentation/atomic_t: Emphasize that failed atomic operations give no ordering
The ORDERING section of Documentation/atomic_t.txt can easily be read as
saying that conditional atomic RMW operations that fail are ordered when
those operations have the _acquire() or _release() suffixes.  This is
not the case, therefore update this section to make it clear that failed
conditional atomic RMW operations provide no ordering.

Reported-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
2024-05-06 14:29:04 -07:00
Kushagra Verma
aae0c8a50d Documentation: Fixed a typo in atomic_t.txt
Fixed a typo in the word 'architecture'.

Signed-off-by: Kushagra Verma <kushagra765@outlook.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2023-01-03 20:47:04 -08:00
Peter Zijlstra
55bccf1f93 Documentation/atomic_t: Document forward progress expectations
Add a few words on forward progress; there's been quite a bit of
confusion on the subject.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lkml.kernel.org/r/YQK9ziyogxTH0m9H@hirez.programming.kicks-ass.net
2021-08-04 15:16:47 +02:00
Peter Zijlstra
d1bbfd0c7c Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()
There seems to be a significant amount of confusion around the new
try_cmpxchg(), despite this being more like the C11
atomic_compare_exchange_*() family. Add a few words of clarification
on how cmpxchg() and try_cmpxchg() relate to one another.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@hirez.programming.kicks-ass.net
2021-07-07 13:53:25 +02:00
Boqun Feng
e30d023555 Documentation/litmus-tests/atomic: Add a test for smp_mb__after_atomic()
We already use a litmus test in atomic_t.txt to describe atomic RMW +
smp_mb__after_atomic() is stronger than acquire (both the read and the
write parts are ordered). So make it a litmus test in atomic-tests
directory, so that people can access the litmus easily.

Additionally, change the processor numbers "P1, P2" to "P0, P1" in
atomic_t.txt for the consistency with the processor numbers in the
litmus test, which herd can handle.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-06-29 12:05:18 -07:00
Boqun Feng
4dcd4d36dd Documentation/litmus-tests/atomic: Add a test for atomic_set()
We already use a litmus test in atomic_t.txt to describe the behavior of
an atomic_set() with the an atomic RMW, so add it into atomic-tests
directory to make it easily accessible for anyone who cares about the
semantics of our atomic APIs.

Besides currently the litmus test "atomic-set" in atomic_t.txt has a few
things to be improved:

1)	The CPU/Processor numbers "P1,P2" are not only inconsistent with
	the rest of the document, which uses "CPU0" and "CPU1", but also
	unacceptable by the herd tool, which requires processors start
	at "P0".

2)	The initialization block uses a "atomic_set()", which is OK, but
	it's better to use ATOMIC_INIT() to make clear this is an
	initialization.

3)	The return value of atomic_add_unless() is discarded
	inexplicitly, which is OK for C language, but it will be helpful
	to the herd tool if we use a void cast to make the discard
	explicit.

4)	The name and the paragraph describing the test need to be more
	accurate and aligned with our wording in LKMM.

Therefore fix these in both atomic_t.txt and the new added litmus test.

Acked-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-06-29 12:05:18 -07:00
Linus Torvalds
e192832869 Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking updates from Ingo Molnar:
 "The main changes in this cycle are:

   - rwsem scalability improvements, phase #2, by Waiman Long, which are
     rather impressive:

       "On a 2-socket 40-core 80-thread Skylake system with 40 reader
        and writer locking threads, the min/mean/max locking operations
        done in a 5-second testing window before the patchset were:

         40 readers, Iterations Min/Mean/Max = 1,807/1,808/1,810
         40 writers, Iterations Min/Mean/Max = 1,807/50,344/151,255

        After the patchset, they became:

         40 readers, Iterations Min/Mean/Max = 30,057/31,359/32,741
         40 writers, Iterations Min/Mean/Max = 94,466/95,845/97,098"

     There's a lot of changes to the locking implementation that makes
     it similar to qrwlock, including owner handoff for more fair
     locking.

     Another microbenchmark shows how across the spectrum the
     improvements are:

       "With a locking microbenchmark running on 5.1 based kernel, the
        total locking rates (in kops/s) on a 2-socket Skylake system
        with equal numbers of readers and writers (mixed) before and
        after this patchset were:

        # of Threads   Before Patch      After Patch
        ------------   ------------      -----------
             2            2,618             4,193
             4            1,202             3,726
             8              802             3,622
            16              729             3,359
            32              319             2,826
            64              102             2,744"

     The changes are extensive and the patch-set has been through
     several iterations addressing various locking workloads. There
     might be more regressions, but unless they are pathological I
     believe we want to use this new implementation as the baseline
     going forward.

   - jump-label optimizations by Daniel Bristot de Oliveira: the primary
     motivation was to remove IPI disturbance of isolated RT-workload
     CPUs, which resulted in the implementation of batched jump-label
     updates. Beyond the improvement of the real-time characteristics
     kernel, in one test this patchset improved static key update
     overhead from 57 msecs to just 1.4 msecs - which is a nice speedup
     as well.

   - atomic64_t cross-arch type cleanups by Mark Rutland: over the last
     ~10 years of atomic64_t existence the various types used by the
     APIs only had to be self-consistent within each architecture -
     which means they became wildly inconsistent across architectures.
     Mark puts and end to this by reworking all the atomic64
     implementations to use 's64' as the base type for atomic64_t, and
     to ensure that this type is consistently used for parameters and
     return values in the API, avoiding further problems in this area.

   - A large set of small improvements to lockdep by Yuyang Du: type
     cleanups, output cleanups, function return type and othr cleanups
     all around the place.

   - A set of percpu ops cleanups and fixes by Peter Zijlstra.

   - Misc other changes - please see the Git log for more details"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (82 commits)
  locking/lockdep: increase size of counters for lockdep statistics
  locking/atomics: Use sed(1) instead of non-standard head(1) option
  locking/lockdep: Move mark_lock() inside CONFIG_TRACE_IRQFLAGS && CONFIG_PROVE_LOCKING
  x86/jump_label: Make tp_vec_nr static
  x86/percpu: Optimize raw_cpu_xchg()
  x86/percpu, sched/fair: Avoid local_clock()
  x86/percpu, x86/irq: Relax {set,get}_irq_regs()
  x86/percpu: Relax smp_processor_id()
  x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()
  locking/rwsem: Guard against making count negative
  locking/rwsem: Adaptive disabling of reader optimistic spinning
  locking/rwsem: Enable time-based spinning on reader-owned rwsem
  locking/rwsem: Make rwsem->owner an atomic_long_t
  locking/rwsem: Enable readers spinning on writer
  locking/rwsem: Clarify usage of owner's nonspinaable bit
  locking/rwsem: Wake up almost all readers in wait queue
  locking/rwsem: More optimal RT task handling of null owner
  locking/rwsem: Always release wait_lock before waking up tasks
  locking/rwsem: Implement lock handoff to prevent lock starvation
  locking/rwsem: Make rwsem_spin_on_owner() return owner state
  ...
2019-07-08 16:12:03 -07:00
Alan Stern
2966f8d440 Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()
The description of smp_mb__before_atomic() and smp_mb__after_atomic()
in Documentation/atomic_t.txt is slightly terse and misleading.  It
does not clearly state which other instructions are ordered by these
barriers.

This improves the text to make the actual ordering implications clear,
and also to explain how these barriers differ from a RELEASE or
ACQUIRE ordering.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-06-19 09:31:53 -07:00
Peter Zijlstra
69d927bba3 x86/atomic: Fix smp_mb__{before,after}_atomic()
Recent probing at the Linux Kernel Memory Model uncovered a
'surprise'. Strongly ordered architectures where the atomic RmW
primitive implies full memory ordering and
smp_mb__{before,after}_atomic() are a simple barrier() (such as x86)
fail for:

	*x = 1;
	atomic_inc(u);
	smp_mb__after_atomic();
	r0 = *y;

Because, while the atomic_inc() implies memory order, it
(surprisingly) does not provide a compiler barrier. This then allows
the compiler to re-order like so:

	atomic_inc(u);
	*x = 1;
	smp_mb__after_atomic();
	r0 = *y;

Which the CPU is then allowed to re-order (under TSO rules) like:

	atomic_inc(u);
	r0 = *y;
	*x = 1;

And this very much was not intended. Therefore strengthen the atomic
RmW ops to include a compiler barrier.

NOTE: atomic_{or,and,xor} and the bitops already had the compiler
barrier.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-06-17 12:09:59 +02:00
Peter Zijlstra
fff9b6c7d2 Documentation/atomic_t.txt: Clarify pure non-rmw usage
Clarify that pure non-RMW usage of atomic_t is pointless, there is
nothing 'magical' about atomic_set() / atomic_read().

This is something that seems to confuse people, because I happen upon it
semi-regularly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190524115231.GN2623@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-06-03 12:32:57 +02:00
Peter Zijlstra
f1887143f5 Documentation/atomic_t: Clarify signed vs unsigned
Clarify the whole signed vs unsigned issue for atomic_t.

There has been enough confusion on this topic to warrant a few explicit
words I feel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-03-18 10:27:52 -07:00
Peter Zijlstra
ca110694c6 Documentation/locking/atomic: Finish the document...
Julia reported that the document looked unfinished, and it is. I
forgot to include the example cooked up by Paul here:

  https://lkml.kernel.org/r/20170731174345.GL3730@linux.vnet.ibm.com

and I added an explicit example showing how, while it is an ACQUIRE
pattern, it really does provide an MB.

Reported-by: Julia Cartwright <julia@ni.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-08-25 11:06:33 +02:00
Peter Zijlstra
706eeb3e9c Documentation/locking/atomic: Add documents for new atomic_t APIs
Since we've vastly expanded the atomic_t interface in recent years the
existing documentation is woefully out of date and people seem to get
confused a bit.

Start a new document to hopefully better explain the current state of
affairs.

The old atomic_ops.txt also covers bitmaps and a few more details so
this is not a full replacement and we'll therefore keep that document
around until such a time that we've managed to write more text to cover
its entire.

Also please, ReST people, go away.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-08-10 12:29:00 +02:00