diff options
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/xfs/xfs_buf_item.c | 88 |
1 files changed, 65 insertions, 23 deletions
diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c index df7322ed73fa..023d4e0385dd 100644 --- a/fs/xfs/xfs_buf_item.c +++ b/fs/xfs/xfs_buf_item.c @@ -452,10 +452,18 @@ xfs_buf_item_format( * This is called to pin the buffer associated with the buf log item in memory * so it cannot be written out. * - * We also always take a reference to the buffer log item here so that the bli - * is held while the item is pinned in memory. This means that we can - * unconditionally drop the reference count a transaction holds when the - * transaction is completed. + * We take a reference to the buffer log item here so that the BLI life cycle + * extends at least until the buffer is unpinned via xfs_buf_item_unpin() and + * inserted into the AIL. + * + * We also need to take a reference to the buffer itself as the BLI unpin + * processing requires accessing the buffer after the BLI has dropped the final + * BLI reference. See xfs_buf_item_unpin() for an explanation. + * If unpins race to drop the final BLI reference and only the + * BLI owns a reference to the buffer, then the loser of the race can have the + * buffer fgreed from under it (e.g. on shutdown). Taking a buffer reference per + * pin count ensures the life cycle of the buffer extends for as + * long as we hold the buffer pin reference in xfs_buf_item_unpin(). */ STATIC void xfs_buf_item_pin( @@ -470,13 +478,30 @@ xfs_buf_item_pin( trace_xfs_buf_item_pin(bip); + xfs_buf_hold(bip->bli_buf); atomic_inc(&bip->bli_refcount); atomic_inc(&bip->bli_buf->b_pin_count); } /* - * This is called to unpin the buffer associated with the buf log item which - * was previously pinned with a call to xfs_buf_item_pin(). + * This is called to unpin the buffer associated with the buf log item which was + * previously pinned with a call to xfs_buf_item_pin(). We enter this function + * with a buffer pin count, a buffer reference and a BLI reference. + * + * We must drop the BLI reference before we unpin the buffer because the AIL + * doesn't acquire a BLI reference whenever it accesses it. Therefore if the + * refcount drops to zero, the bli could still be AIL resident and the buffer + * submitted for I/O at any point before we return. This can result in IO + * completion freeing the buffer while we are still trying to access it here. + * This race condition can also occur in shutdown situations where we abort and + * unpin buffers from contexts other that journal IO completion. + * + * Hence we have to hold a buffer reference per pin count to ensure that the + * buffer cannot be freed until we have finished processing the unpin operation. + * The reference is taken in xfs_buf_item_pin(), and we must hold it until we + * are done processing the buffer state. In the case of an abort (remove = + * true) then we re-use the current pin reference as the IO reference we hand + * off to IO failure handling. */ STATIC void xfs_buf_item_unpin( @@ -493,24 +518,18 @@ xfs_buf_item_unpin( trace_xfs_buf_item_unpin(bip); - /* - * Drop the bli ref associated with the pin and grab the hold required - * for the I/O simulation failure in the abort case. We have to do this - * before the pin count drops because the AIL doesn't acquire a bli - * reference. Therefore if the refcount drops to zero, the bli could - * still be AIL resident and the buffer submitted for I/O (and freed on - * completion) at any point before we return. This can be removed once - * the AIL properly holds a reference on the bli. - */ freed = atomic_dec_and_test(&bip->bli_refcount); - if (freed && !stale && remove) - xfs_buf_hold(bp); if (atomic_dec_and_test(&bp->b_pin_count)) wake_up_all(&bp->b_waiters); - /* nothing to do but drop the pin count if the bli is active */ - if (!freed) + /* + * Nothing to do but drop the buffer pin reference if the BLI is + * still active. + */ + if (!freed) { + xfs_buf_rele(bp); return; + } if (stale) { ASSERT(bip->bli_flags & XFS_BLI_STALE); @@ -523,6 +542,15 @@ xfs_buf_item_unpin( trace_xfs_buf_item_unpin_stale(bip); /* + * The buffer has been locked and referenced since it was marked + * stale so we own both lock and reference exclusively here. We + * do not need the pin reference any more, so drop it now so + * that we only have one reference to drop once item completion + * processing is complete. + */ + xfs_buf_rele(bp); + + /* * If we get called here because of an IO error, we may or may * not have the item on the AIL. xfs_trans_ail_delete() will * take care of that situation. xfs_trans_ail_delete() drops @@ -538,16 +566,30 @@ xfs_buf_item_unpin( ASSERT(bp->b_log_item == NULL); } xfs_buf_relse(bp); - } else if (remove) { + return; + } + + if (remove) { /* - * The buffer must be locked and held by the caller to simulate - * an async I/O failure. We acquired the hold for this case - * before the buffer was unpinned. + * We need to simulate an async IO failures here to ensure that + * the correct error completion is run on this buffer. This + * requires a reference to the buffer and for the buffer to be + * locked. We can safely pass ownership of the pin reference to + * the IO to ensure that nothing can free the buffer while we + * wait for the lock and then run the IO failure completion. */ xfs_buf_lock(bp); bp->b_flags |= XBF_ASYNC; xfs_buf_ioend_fail(bp); + return; } + + /* + * BLI has no more active references - it will be moved to the AIL to + * manage the remaining BLI/buffer life cycle. There is nothing left for + * us to do here so drop the pin reference to the buffer. + */ + xfs_buf_rele(bp); } STATIC uint |