On Wed, Feb 06, 2013 at 07:44:40AM -0500, Brian Foster wrote: > xfs_force_log() is not safe from all contexts. Add a flag parameter > to xfs_buf_trylock() to specify when the force is appropriate and > create a macro to preserve current behavior. Static inline functions are preferred over macros because they provide type checking..... However, here's what happens when I actually have time to think about the problem at hand - I suggest a completely different fix. Sorry, Brain, I'm not doing this intentionally to make your life harder than it needs to be... :/ My logic (goes back to the history of changes that lead to the log force being in xfs_buf_trylock()) is as follows: 1. xfs_buf_lock() had the log force added originally because _xfs_buf_find was getting stuck on locked, pinned stale buffers. This was a result of the rework of the busy extent handling to avoid synchronous transactions and/or log forces when reusing recently freed blocks. Somewhere a log force was needed to get the freeing transaction on disk before reuse occurred, and that was done on demand in xfs_buf_lock() when such a stale buffer was being used after a subsequent allocation. 2a. xfs_inode_item_push() used to flush inodes to the the underlying buffer and would skip the flush if the underlying buffer was locked. Hence pinned inodes could not be flushed if the underlying buffer was locked (i.e. when pinned and stale). 2b. xfs_buf_item_push() would fail to lock the buffer if it raced with pinning and do nothing. 3. this resulted in neither xfs_inode_item_push() or xfs_buf_item_push() telling the xfsaild to issue a log force, the AIL would stop doing work as finished it's traverse, and hence tranaction reservations stalled until something else issued a log force and unpinned the tail of the log. 4. the simple and obvious fix at the time was to have xfs_buf_trylock() do the same as xfs_buf_lock() and force the log when a buffer in the state that caused the stall was detected. <time passes> 5a. delayed write buffers were reworked, completely changing how inode flushing and hence xfs_inode_item_push() worked. 5b. delayed write buffers were reworked, changing how xfs_buf_item_push() worked. 6. several bugs in pinned/stale item handling were fixed, and now both xfs_inode_item_push() and xfs_buf_item_push() detect pinned inodes and buffers and tell the xfsaild to force the log appropriately. This means xfs_buf_trylock() no longer needs to force the log to provide a get-out-of-xfsaild-stall-free mechanism. 7. This log force while holding the ail lock bug is discovered. So what I'm thinking is that as a result of 5+6, we have no need for the log force in the xfs_buf_trylock code any more. The only place we actually care about locking latency any more is in _xfs_buf_find(), and when the trylock fails there we immediately run xfs_buf_lock() (unless XBF_TRYLOCK was set) and it will do the log force for us. IOWs, I think the log force code in xfs_buf_trylock() is simply redundant and we should just remove it. With the second patch in this series, racing on the buffer being pinned will now do the right thing (i.e. trigger a log force via the xfsaild) and so we end up both simplifying the code and fixing the bug.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
