• Steven Rostedt (Red Hat)'s avatar
    ring-buffer: Fix infinite spin in reading buffer · 24607f11
    Steven Rostedt (Red Hat) authored
    Commit 651e22f2 "ring-buffer: Always reset iterator to reader page"
    fixed one bug but in the process caused another one. The reset is to
    update the header page, but that fix also changed the way the cached
    reads were updated. The cache reads are used to test if an iterator
    needs to be updated or not.
    
    A ring buffer iterator, when created, disables writes to the ring buffer
    but does not stop other readers or consuming reads from happening.
    Although all readers are synchronized via a lock, they are only
    synchronized when in the ring buffer functions. Those functions may
    be called by any number of readers. The iterator continues down when
    its not interrupted by a consuming reader. If a consuming read
    occurs, the iterator starts from the beginning of the buffer.
    
    The way the iterator sees that a consuming read has happened since
    its last read is by checking the reader "cache". The cache holds the
    last counts of the read and the reader page itself.
    
    Commit 651e22f2 changed what was saved by the cache_read when
    the rb_iter_reset() occurred, making the iterator never match the cache.
    Then if the iterator calls rb_iter_reset(), it will go into an
    infinite loop by checking if the cache doesn't match, doing the reset
    and retrying, just to see that the cache still doesn't match! Which
    should never happen as the reset is suppose to set the cache to the
    current value and there's locks that keep a consuming reader from
    having access to the data.
    
    Fixes: 651e22f2
    
     "ring-buffer: Always reset iterator to reader page"
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    24607f11