On Mon, Feb 6, 2017 at 4:30 AM, Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx> wrote:
Hi all,
Storage/posix does syscalls on backend filesystem holding inode->lock. This is bad as it is a lock global to the inode and can cause unnecessary contention with unrelated translators doing unrelated operations (like inode_ctx_get etc). I've discussed one such issue in [2]. A quick git grep on "inode->lock" in storage/posix gave following results:
* posix_writev -
GLUSTERFS_WRITE_IS_APPEND - looks like used by arbiter/afr.
GLUSTERFS_WRITE_UPDATE_ATOMIC - looks like used by shard
* posix_fd_ctx_get - resolves gfid handle (which can involve multiple readlinks and lstats) in inode->lock. Though the cost is only once when fd-ctx is freshly created.
* code that maintains pgfid xattrs - executed in various dentry modification fops like mkdir, create, mknod, unlink, rename, link etc
* code that uses GET_LINK_COUNT - looks like used by shard and EC. Note that this code is executed during rename/unlink.
* posix_create_link_if_gfid_exists - looks like used by afr entry selfheal
* posix_(f)xattrop - various xlators like afr, marker during different fops.
The question here is can we've synchronization using a lock visible only to storage/posix so that contention is localized (like [1])?
I think the answer depends on the degree of isolation required across threads operating on the same inode. If the operations being done within "inode->lock" do not cause any side effects elsewhere in the xlator stack, we should be able to replace inode->lock with a more local lock. At the outset it looks like we should be able to synchronize using a smaller lock for most cases. A more careful analysis is needed to determine if there are scenarios where inode->lock helps.
Regards,
Vijay
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