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    SubjectRe: sem_lock() vs qspinlocks
    On Fri, May 20, 2016 at 05:21:49PM +0200, Peter Zijlstra wrote:

    > Let me write a patch..

    OK, something like the below then.. lemme go build that and verify that
    too fixes things.

    ---
    Subject: locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()

    Similar to commits:

    51d7d5205d33 ("powerpc: Add smp_mb() to arch_spin_is_locked()")
    d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")

    qspinlock suffers from the fact that the _Q_LOCKED_VAL store is
    unordered inside the ACQUIRE of the lock.

    And while this is not a problem for the regular mutual exclusive
    critical section usage of spinlocks, it breaks creative locking like:

    spin_lock(A) spin_lock(B)
    spin_unlock_wait(B) if (!spin_is_locked(A))
    do_something() do_something()

    In that both CPUs can end up running do_something at the same time,
    because our _Q_LOCKED_VAL store can drop past the spin_unlock_wait()
    spin_is_locked() loads (even on x86!!).

    To avoid making the normal case slower, add smp_mb()s to the less used
    spin_unlock_wait() / spin_is_locked() side of things to avoid this
    problem.

    Reported-by: Davidlohr Bueso <dave@stgolabs.net>
    Reported-by: Giovanni Gherdovich <ggherdovich@suse.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    include/asm-generic/qspinlock.h | 27 ++++++++++++++++++++++++++-
    1 file changed, 26 insertions(+), 1 deletion(-)

    diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
    index 35a52a880b2f..6bd05700d8c9 100644
    --- a/include/asm-generic/qspinlock.h
    +++ b/include/asm-generic/qspinlock.h
    @@ -28,7 +28,30 @@
    */
    static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
    {
    - return atomic_read(&lock->val);
    + /*
    + * queued_spin_lock_slowpath() can ACQUIRE the lock before
    + * issuing the unordered store that sets _Q_LOCKED_VAL.
    + *
    + * See both smp_cond_acquire() sites for more detail.
    + *
    + * This however means that in code like:
    + *
    + * spin_lock(A) spin_lock(B)
    + * spin_unlock_wait(B) spin_is_locked(A)
    + * do_something() do_something()
    + *
    + * Both CPUs can end up running do_something() because the store
    + * setting _Q_LOCKED_VAL will pass through the loads in
    + * spin_unlock_wait() and/or spin_is_locked().
    + *
    + * Avoid this by issuing a full memory barrier between the spin_lock()
    + * and the loads in spin_unlock_wait() and spin_is_locked().
    + *
    + * Note that regular mutual exclusion doesn't care about this
    + * delayed store.
    + */
    + smp_mb();
    + return atomic_read(&lock->val) & _Q_LOCKED_MASK;
    }

    /**
    @@ -108,6 +131,8 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock)
    */
    static inline void queued_spin_unlock_wait(struct qspinlock *lock)
    {
    + /* See queued_spin_is_locked() */
    + smp_mb();
    while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
    cpu_relax();
    }
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      Last update: 2016-05-20 18:21    [W:2.930 / U:0.084 seconds]
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