• Paul E. McKenney's avatar
    rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock() · 1f3ebc82
    Paul E. McKenney authored
    Commit bb73c52b ("rcu: Don't disable preemption for Tiny and Tree
    RCU readers") removed the barrier() calls from rcu_read_lock() and
    rcu_write_lock() in CONFIG_PREEMPT=n&&CONFIG_PREEMPT_COUNT=n kernels.
    Within RCU, this commit was OK, but it failed to account for things like
    get_user() that can pagefault and that can be reordered by the compiler.
    Lack of the barrier() calls in rcu_read_lock() and rcu_read_unlock()
    can cause these page faults to migrate into RCU read-side critical
    sections, which in CONFIG_PREEMPT=n kernels could result in too-short
    grace periods and arbitrary misbehavior.  Please see commit 386afc91
    ("spinlocks and preemption points need to be at least compiler barriers")
    and Linus's commit 66be4e66 ("rcu: locking and unlocking need to
    always be at least barriers"), this last of which restores the barrier()
    call to both rcu_read_lock() and rcu_read_unlock().
    
    This commit removes barrier() calls that are no longer needed given that
    the addition of them in Linus's commit noted above.  The combination of
    this commit and Linus's commit effectively reverts commit bb73c52b
    ("rcu: Don't disable preemption for Tiny and Tree RCU readers").
    Reported-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
    [ paulmck: Fix embarrassing typo located by Alan Stern. ]
    1f3ebc82
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