lkml.org 
[lkml]   [2014]   [Jun]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    SubjectRe: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks
    From
    Date
    On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote:
    > If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at
    > the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock,
    > so, in this case, cmpxchg or xchg isn't really atomic at all.

    So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at
    the same time, would the below change address this problem?

    -----
    diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
    index 838dc9e..8396721 100644
    --- a/kernel/locking/mcs_spinlock.c
    +++ b/kernel/locking/mcs_spinlock.c
    @@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock)
    if (likely(prev == NULL))
    return true;

    - ACCESS_ONCE(prev->next) = node;
    + xchg(&prev->next, node);

    /*
    * Normally @prev is untouchable after the above store; because at that
    @@ -144,7 +144,7 @@ unqueue:
    */

    ACCESS_ONCE(next->prev) = prev;
    - ACCESS_ONCE(prev->next) = next;
    + xchg(&prev->next, next);

    return false;
    }



    \
     
     \ /
      Last update: 2014-06-02 20:21    [W:3.311 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site