rwsem: simplify __rwsem_do_wake

This is mostly for cleanup value:

- We don't need several gotos to handle the case where the first
  waiter is a writer. Two simple tests will do (and generate very
  similar code).

- In the remainder of the function, we know the first waiter is a reader,
  so we don't have to double check that. We can use do..while loops
  to iterate over the readers to wake (generates slightly better code).

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: implement support for write lock stealing on the fastpath

When we decide to wake up readers, we must first grant them as many
read locks as necessary, and then actually wake up all these readers.
But in order to know how many read shares to grant, we must first
count the readers at the head of the queue. This might take a while
if there are many readers, and we want to be protected against a
writer stealing the lock while we're counting. To that end, we grant
the first reader lock before counting how many more readers are queued.

We also require some adjustments to the wake_type semantics.

RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to
be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free
as nobody could steal it while we hold the wait_lock. This doesn't
make sense once we implement fastpath write lock stealing, so we now
use RWSEM_WAKE_ANY in that case.

Similarly, when rwsem_down_write_failed found that a read lock was active,
it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could
be woken without checking first that the rwsem was available. We can't do
that anymore since the existing readers might release their read locks,
and a writer could steal the lock before we wake up additional readers.
So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want
to wake readers, but we don't currently hold any read lock.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: do not block readers at head of queue if other readers are active

This change fixes a race condition where a reader might determine it
needs to block, but by the time it acquires the wait_lock the rwsem
has active readers and no queued waiters.

In this situation the reader can just in parallel with the existing active
readers; it does not need to block until the active readers complete.

Thanks to Peter Hurley for noticing this possible race.

Signed-off-by: Michel Lespinasse <walken@google.com>
lib: Update RWSEM to Linux 3.10

Squashed commit of the following:

commit 28f9a942e4e1578c0e26e8211ba49bdf520de95c
Author: Davidlohr Bueso <davidlohr.bueso@hp.com>
Date:   Tue May 7 06:46:02 2013 -0700

    rwsem: no need for explicit signed longs

    Change explicit "signed long" declarations into plain "long" as suggested
    by Peter Hurley.

    Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Reviewed-by: Michel Lespinasse <walken@google.com>
    Signed-off-by: Michel Lespinasse <walken@google.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 353ad88b6af95bc7bb127b3b5b1cfa36c72a7b67
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:46:00 2013 -0700

    rwsem: do not block readers at head of queue if other readers are active

    This change fixes a race condition where a reader might determine it
    needs to block, but by the time it acquires the wait_lock the rwsem has
    active readers and no queued waiters.

    In this situation the reader can run in parallel with the existing
    active readers; it does not need to block until the active readers
    complete.

    Thanks to Peter Hurley for noticing this possible race.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit e9149aa93e4e6919da4fe9e77a27f9c6a0159463
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:59 2013 -0700

    rwsem: implement support for write lock stealing on the fastpath

    When we decide to wake up readers, we must first grant them as many read
    locks as necessary, and then actually wake up all these readers.  But in
    order to know how many read shares to grant, we must first count the
    readers at the head of the queue.  This might take a while if there are
    many readers, and we want to be protected against a writer stealing the
    lock while we're counting.  To that end, we grant the first reader lock
    before counting how many more readers are queued.

    We also require some adjustments to the wake_type semantics.

    RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be
    RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as
    nobody could steal it while we hold the wait_lock.  This doesn't make
    sense once we implement fastpath write lock stealing, so we now use
    RWSEM_WAKE_ANY in that case.

    Similarly, when rwsem_down_write_failed found that a read lock was
    active, it would use RWSEM_WAKE_READ_OWNED which signalled that new
    readers could be woken without checking first that the rwsem was
    available.  We can't do that anymore since the existing readers might
    release their read locks, and a writer could steal the lock before we
    wake up additional readers.  So, we have to use a new RWSEM_WAKE_READERS
    value to indicate we only want to wake readers, but we don't currently
    hold any read lock.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 20addabcb540eb4b4e11d7e8af037543767e081d
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:58 2013 -0700

    rwsem: simplify __rwsem_do_wake

    This is mostly for cleanup value:

    - We don't need several gotos to handle the case where the first
      waiter is a writer. Two simple tests will do (and generate very
      similar code).

    - In the remainder of the function, we know the first waiter is a reader,
      so we don't have to double check that. We can use do..while loops
      to iterate over the readers to wake (generates slightly better code).

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit e56c8eea2628841a06a2420e2c6ca8b8c6bd4434
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:57 2013 -0700

    rwsem: skip initial trylock in rwsem_down_write_failed

    We can skip the initial trylock in rwsem_down_write_failed() if there
    are known active lockers already, thus saving one likely-to-fail
    cmpxchg.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Acked-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 286040cc550df96bef6e97a5bc0231ded9a0187b
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:56 2013 -0700

    rwsem: avoid taking wait_lock in rwsem_down_write_failed

    In rwsem_down_write_failed(), if there are active locks after we wake up
    (i.e.  the lock got stolen from us), skip taking the wait_lock and go
    back to sleep immediately.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Acked-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 6c7229b31a4c099157242c58f5d33ce728daff4b
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:55 2013 -0700

    rwsem: use cmpxchg for trying to steal write lock

    Using rwsem_atomic_update to try stealing the write lock forced us to
    undo the adjustment in the failure path.  We can have simpler and faster
    code by using cmpxchg instead.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Acked-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 85dcaeae08b5738f8ea0b08734ab97e57bb9e8e2
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:54 2013 -0700

    rwsem: more agressive lock stealing in rwsem_down_write_failed

    Some small code simplifications can be achieved by doing more agressive
    lock stealing:

    - When rwsem_down_write_failed() notices that there are no active locks
      (and thus no thread to wake us if we decided to sleep), it used to wake
      the first queued process. However, stealing the lock is also sufficient
      to deal with this case, so we don't need this check anymore.

    - In try_get_writer_sem(), we can steal the lock even when the first waiter
      is a reader. This is correct because the code path that wakes readers is
      protected by the wait_lock. As to the performance effects of this change,
      they are expected to be minimal: readers are still granted the lock
      (rather than having to acquire it themselves) when they reach the front
      of the wait queue, so we have essentially the same behavior as in
      rwsem-spinlock.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 082a2e9c40e01d70c59ca4e3a9b552b81d573332
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:53 2013 -0700

    rwsem: simplify rwsem_down_write_failed

    When waking writers, we never grant them the lock - instead, they have
    to acquire it themselves when they run, and remove themselves from the
    wait_list when they succeed.

    As a result, we can do a few simplifications in rwsem_down_write_failed():

    - We don't need to check for !waiter.task since __rwsem_do_wake() doesn't
      remove writers from the wait_list

    - There is no point releaseing the wait_lock before entering the wait loop,
      as we will need to reacquire it immediately. We can change the loop so
      that the lock is always held at the start of each loop iteration.

    - We don't need to get a reference on the task structure, since the task
      is responsible for removing itself from the wait_list. There is no risk,
      like in the rwsem_down_read_failed() case, that a task would wake up and
      exit (thus destroying its task structure) while __rwsem_do_wake() is
      still running - wait_lock protects against that.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 6b3306dccd59d5dcf22e55a27827bccf51a796e1
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:52 2013 -0700

    rwsem: simplify rwsem_down_read_failed

    When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS
    adjustment doesn't cause other readers to block, so we never have to
    worry about waking them back after canceling this adjustment in
    rwsem_down_read_failed().

    We also never want to steal the lock in rwsem_down_read_failed(), so we
    don't have to grab the wait_lock either.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit c6b315c32d1ffbb33c75ab5e0f8ee27c89d206c4
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:51 2013 -0700

    rwsem: move rwsem_down_failed_common code into rwsem_down_{read,write}_failed

    Remove the rwsem_down_failed_common function and replace it with two
    identical copies of its code in rwsem_down_{read,write}_failed.

    This is because we want to make different optimizations in
    rwsem_down_{read,write}_failed; we are adding this pure-duplication
    step as a separate commit in order to make it easier to check the
    following steps.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 7d2869248ccd7971fc1b79e4bf22cb4be446faf4
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:50 2013 -0700

    rwsem: shorter spinlocked section in rwsem_down_failed_common()

    This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
    sections in rwsem_down_failed_common():

    - We only need the sem->wait_lock to insert ourselves on the wait_list;
      the waiter node can be prepared outside of the wait_lock.

    - The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
      before checking if we actually need to sleep; it doesn't need to protect
      the entire function.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 8c163f8ddf5c0bd018706dbdd44fb96a073401d0
Author: Michel Lespinasse <walken@google.com>
Date:   Tue May 7 06:45:49 2013 -0700

    rwsem: make the waiter type an enumeration rather than a bitmask

    We are not planning to add some new waiter flags, so we can convert the
    waiter type into an enumeration.

    Background: David Howells suggested I do this back when I tried adding
    a new waiter type for unfair readers. However, I believe the cleanup
    applies regardless of that use case.

    Signed-off-by: Michel Lespinasse <walken@google.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
    Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

commit 9a126edbed8ccfedda04d5c3fe256cd139d4bcca
Author: Alex Shi <alex.shi@intel.com>
Date:   Tue Feb 5 21:11:55 2013 +0800

    rwsem: Implement writer lock-stealing for better scalability

    Commit 5a505085f043 ("mm/rmap: Convert the struct anon_vma::mutex
    to an rwsem") changed struct anon_vma::mutex to an rwsem, which
    caused aim7 fork_test performance to drop by 50%.

    Yuanhan Liu did the following excellent analysis:

        https://lkml.org/lkml/2013/1/29/84

    and found that the regression is caused by strict, serialized,
    FIFO sequential write-ownership of rwsems. Ingo suggested
    implementing opportunistic lock-stealing for the front writer
    task in the waitqueue.

    Yuanhan Liu implemented lock-stealing for spinlock-rwsems,
    which indeed recovered much of the regression - confirming
    the analysis that the main factor in the regression was the
    FIFO writer-fairness of rwsems.

    In this patch we allow lock-stealing to happen when the first
    waiter is also writer. With that change in place the
    aim7 fork_test performance is fully recovered on my
    Intel NHM EP, NHM EX, SNB EP 2S and 4S test-machines.

    Reported-by: lkp@linux.intel.com
    Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
    Signed-off-by: Alex Shi <alex.shi@intel.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Anton Blanchard <anton@samba.org>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: paul.gortmaker@windriver.com
    Link: https://lkml.org/lkml/2013/1/29/84
    Link: http://lkml.kernel.org/r/1360069915-31619-1-git-send-email-alex.shi@intel.com
    [ Small stylistic fixes, updated changelog. ]
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

commit 77302ff69a93cfba32aaee98d2bb864ba8886e40
Author: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Date:   Fri Feb 1 18:59:16 2013 +0800

    rwsem-spinlock: Implement writer lock-stealing for better scalability

    We (Linux Kernel Performance project) found a regression
    introduced by commit:

      5a505085f043 mm/rmap: Convert the struct anon_vma::mutex to an rwsem

    which converted all anon_vma::mutex locks rwsem write locks.

    The semantics are the same, but the behavioral difference is
    quite huge in some cases. After investigating it we found the
    root cause: mutexes support lock stealing while rwsems don't.

    Here is the link for the detailed regression report:

      https://lkml.org/lkml/2013/1/29/84

    Ingo suggested adding write lock stealing to rwsems:

        "I think we should allow lock-steal between rwsem writers - that
         will not hurt fairness as most rwsem fairness concerns relate to
         reader vs. writer fairness"

    And here is the rwsem-spinlock version.

    With this patch, we got a double performance increase in one
    test box with following aim7 workfile:

        FILESIZE: 1M
        POOLSIZE: 10M
        10 fork_test

     /usr/bin/time output w/o patch                       /usr/bin/time_output with patch
     -- Percent of CPU this job got: 369%                 Percent of CPU this job got: 537%
     Voluntary context switches: 640595016                Voluntary context switches: 157915561

    We got a 45% increase in CPU usage and saved about 3/4 voluntary context switches.

    Reported-by: LKP project <lkp@linux.intel.com>
    Suggested-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
    Cc: Alex Shi <alex.shi@intel.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Anton Blanchard <anton@samba.org>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: paul.gortmaker@windriver.com
    Link: http://lkml.kernel.org/r/1359716356-23865-1-git-send-email-yuanhan.liu@linux.intel.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

commit 95f8b6683b3f83e40a8ab0d93a9d155954b64396
Author: Andrei F <luxneb@gmail.com>
Date:   Wed Aug 21 21:49:39 2013 +0400

    ARM: rwsem: Update header from initial implementation

    Cleanup due to:

    c16a87ce063f79e0ec7d25ce2950e1bc6db03c72 rwsem: Cleanup includes
    1c8ed640d918290ddc1de5ada02ef6686a733c9f rwsem: Move duplicate struct rwsem declaration to linux/rwsem.h
    12249b34414dba7f386aadcf6be7ca36c6878300 rwsem: Move duplicate init macros and functions to linux/rwsem.h
    41e5887fa39ab272d9266a09cbefdef270e28b93 rwsem: Unify the duplicate rwsem_is_locked() inlines
    aac72277fda6ef788bb8d5deaa502ce9b9b6e472 rwsem: Move duplicate function prototypes to linux/rwsem.h

Conflicts:
	arch/arm/include/asm/rwsem.h
	lib/rwsem-spinlock.c
	lib/rwsem.c