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Diffstat (limited to 'Documentation/technical/racy-git.txt')
| -rw-r--r-- | Documentation/technical/racy-git.txt | 44 |
1 files changed, 24 insertions, 20 deletions
diff --git a/Documentation/technical/racy-git.txt b/Documentation/technical/racy-git.txt index 53aa0c82c2..4a8be4d144 100644 --- a/Documentation/technical/racy-git.txt +++ b/Documentation/technical/racy-git.txt @@ -1,21 +1,21 @@ -Use of index and Racy git problem +Use of index and Racy Git problem ================================= Background ---------- -The index is one of the most important data structures in git. +The index is one of the most important data structures in Git. It represents a virtual working tree state by recording list of paths and their object names and serves as a staging area to write out the next tree object to be committed. The state is "virtual" in the sense that it does not necessarily have to, and often does not, match the files in the working tree. -There are cases git needs to examine the differences between the +There are cases Git needs to examine the differences between the virtual working tree state in the index and the files in the working tree. The most obvious case is when the user asks `git diff` (or its low level implementation, `git diff-files`) or -`git-ls-files --modified`. In addition, git internally checks +`git-ls-files --modified`. In addition, Git internally checks if the files in the working tree are different from what are recorded in the index to avoid stomping on local changes in them during patch application, switching branches, and merging. @@ -24,16 +24,16 @@ In order to speed up this comparison between the files in the working tree and the index entries, the index entries record the information obtained from the filesystem via `lstat(2)` system call when they were last updated. When checking if they differ, -git first runs `lstat(2)` on the files and compares the result +Git first runs `lstat(2)` on the files and compares the result with this information (this is what was originally done by the `ce_match_stat()` function, but the current code does it in `ce_match_stat_basic()` function). If some of these "cached -stat information" fields do not match, git can tell that the +stat information" fields do not match, Git can tell that the files are modified without even looking at their contents. Note: not all members in `struct stat` obtained via `lstat(2)` are used for this comparison. For example, `st_atime` obviously -is not useful. Currently, git compares the file type (regular +is not useful. Currently, Git compares the file type (regular files vs symbolic links) and executable bits (only for regular files) from `st_mode` member, `st_mtime` and `st_ctime` timestamps, `st_uid`, `st_gid`, `st_ino`, and `st_size` members. @@ -41,15 +41,19 @@ With a `USE_STDEV` compile-time option, `st_dev` is also compared, but this is not enabled by default because this member is not stable on network filesystems. With `USE_NSEC` compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec` -members are also compared, but this is not enabled by default +members are also compared. On Linux, this is not enabled by default because in-core timestamps can have finer granularity than on-disk timestamps, resulting in meaningless changes when an inode is evicted from the inode cache. See commit 8ce13b0 of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git -([PATCH] Sync in core time granuality with filesystems, -2005-01-04). - -Racy git +([PATCH] Sync in core time granularity with filesystems, +2005-01-04). This patch is included in kernel 2.6.11 and newer, but +only fixes the issue for file systems with exactly 1 ns or 1 s +resolution. Other file systems are still broken in current Linux +kernels (e.g. CEPH, CIFS, NTFS, UDF), see +https://lkml.org/lkml/2015/6/9/714 + +Racy Git -------- There is one slight problem with the optimization based on the @@ -67,13 +71,13 @@ timestamp does not change, after this sequence, the cached stat information the index entry records still exactly match what you would see in the filesystem, even though the file `foo` is now different. -This way, git can incorrectly think files in the working tree +This way, Git can incorrectly think files in the working tree are unmodified even though they actually are. This is called -the "racy git" problem (discovered by Pasky), and the entries +the "racy Git" problem (discovered by Pasky), and the entries that appear clean when they may not be because of this problem are called "racily clean". -To avoid this problem, git does two things: +To avoid this problem, Git does two things: . When the cached stat information says the file has not been modified, and the `st_mtime` is the same as (or newer than) @@ -116,7 +120,7 @@ timestamp comparison check done with the former logic anymore. The latter makes sure that the cached stat information for `foo` would never match with the file in the working tree, so later checks by `ce_match_stat_basic()` would report that the index entry -does not match the file and git does not have to fall back on more +does not match the file and Git does not have to fall back on more expensive `ce_modified_check_fs()`. @@ -135,9 +139,9 @@ them, and give the same timestamp to the index file: $ git ls-files | git update-index --stdin $ touch -r .datestamp .git/index -This will make all index entries racily clean. The linux-2.6 -project, for example, there are over 20,000 files in the working -tree. On my Athlon 64 X2 3800+, after the above: +This will make all index entries racily clean. The linux project, for +example, there are over 20,000 files in the working tree. On my +Athlon 64 X2 3800+, after the above: $ /usr/bin/time git diff-files 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k @@ -159,7 +163,7 @@ of the cached stat information. Avoiding runtime penalty ------------------------ -In order to avoid the above runtime penalty, post 1.4.2 git used +In order to avoid the above runtime penalty, post 1.4.2 Git used to have a code that made sure the index file got timestamp newer than the youngest files in the index when there are many young files with the same timestamp as the |