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Chris Mason
b9fa21f3da
commitec35e48b28upstream. refcounts have a generic implementation and an asm optimized one. The generic version has extra debugging to make sure that once a refcount goes to zero, refcount_inc won't increase it. The btrfs delayed inode code wasn't expecting this, and we're tripping over the warnings when the generic refcounts are used. We ended up with this race: Process A Process B btrfs_get_delayed_node() spin_lock(root->inode_lock) radix_tree_lookup() __btrfs_release_delayed_node() refcount_dec_and_test(&delayed_node->refs) our refcount is now zero refcount_add(2) <--- warning here, refcount unchanged spin_lock(root->inode_lock) radix_tree_delete() With the generic refcounts, we actually warn again when process B above tries to release his refcount because refcount_add() turned into a no-op. We saw this in production on older kernels without the asm optimized refcounts. The fix used here is to use refcount_inc_not_zero() to detect when the object is in the middle of being freed and return NULL. This is almost always the right answer anyway, since we usually end up pitching the delayed_node if it didn't have fresh data in it. This also changes __btrfs_release_delayed_node() to remove the extra check for zero refcounts before radix tree deletion. btrfs_get_delayed_node() was the only path that was allowing refcounts to go from zero to one. Fixes:6de5f18e7b("btrfs: fix refcount_t usage when deleting btrfs_delayed_node") Signed-off-by: Chris Mason <clm@fb.com> Reviewed-by: Liu Bo <bo.li.liu@oracle.com> Signed-off-by: David Sterba <dsterba@suse.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.