lkml.org 
[lkml]   [2010]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 1/4] input: Introduce buflock, a one-to-many circular buffer mechanism
    >> +#define buflock_write(bw, buf, size, item)				\
    >> + do { \
    >> + bw.next_head = (bw.head + 1) & ((size) - 1); \
    >> + smp_wmb(); \
    >
    > Why do we need the write barrier here?

    I believe my first answer to this question was foggy indeed, so allow me to go
    again, with a time line:

    Scenario 1, correct write order:

    writer store_next_head store_buf store_head
    reader load_head load_buf load_next_head

    Result: head != next_head, incoherent read detected

    Scenario 2, incorrect write order:

    writer store_buf store_next_head store_head
    reader load_head load_buf load_next_head

    Result: head == next_head, incoherent read not detected

    Based on the assumption that scenario 2 could happen if the smp_wmb() is not
    present, the barrier is needed.

    Henrik


    \
     
     \ /
      Last update: 2010-06-04 18:39    [W:0.027 / U:3.732 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site