lkml.org 
[lkml]   [2015]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers
    On Tue, Oct 06, 2015 at 10:42:04AM -0700, Paul E. McKenney wrote:
    > On Tue, Oct 06, 2015 at 10:16:30AM -0700, Josh Triplett wrote:
    > > On Tue, Oct 06, 2015 at 10:01:01AM -0700, Paul E. McKenney wrote:
    > > > On Tue, Oct 06, 2015 at 09:44:45AM -0700, Josh Triplett wrote:
    > > > > On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote:
    > > > > > From: Boqun Feng <boqun.feng@gmail.com>
    > > > > >
    > > > > > Because preempt_disable() maps to barrier() for non-debug builds,
    > > > > > it forces the compiler to spill and reload registers. Because Tree
    > > > > > RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
    > > > > > barrier() instances generate needless extra code for each instance of
    > > > > > rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree
    > > > > > RCU and bloats Tiny RCU.
    > > > > >
    > > > > > This commit therefore removes the preempt_disable() and preempt_enable()
    > > > > > from the non-preemptible implementations of __rcu_read_lock() and
    > > > > > __rcu_read_unlock(), respectively. However, for debug purposes,
    > > > > > preempt_disable() and preempt_enable() are still invoked if
    > > > > > CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
    > > > > > atomic sections in non-preemptible kernels.
    > > > > >
    > > > > > This is based on an earlier patch by Paul E. McKenney, fixing
    > > > > > a bug encountered in kernels built with CONFIG_PREEMPT=n and
    > > > > > CONFIG_PREEMPT_COUNT=y.
    > > > >
    > > > > This also adds explicit barrier() calls to several internal RCU
    > > > > functions, but the commit message doesn't explain those at all.
    > > >
    > > > To compensate for them being removed from rcu_read_lock() and
    > > > rcu_read_unlock(), but yes, I will update.
    > >
    > > That much seemed clear from the comments, but that doesn't explain *why*
    > > those functions need barriers of their own even though rcu_read_lock()
    > > and rcu_read_unlock() don't.
    >
    > Ah. The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act
    > by implicitly extending (and, if need be, merging) the RCU read-side
    > critical sections to include all the code between successive quiescent
    > states, for example, all the code between a pair of calls to schedule().
    >
    > Therefore, there need to be barrier() calls in the quiescent-state
    > functions. Some could be argued to be implicitly present due to
    > translation-unit boundaries, but paranoia and all that.
    >
    > Would adding that sort of explanation help?

    Yes, it would.


    \
     
     \ /
      Last update: 2015-10-06 20:21    [W:6.226 / U:0.016 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site