Re: What's cooking in git.git (Nov 2016, #06; Mon, 28)

[Jeff King <peff@peff.net>]

On Wed, Nov 30, 2016 at 06:59:52PM -0500, Jeff King wrote:

> So I think there is some other chdir(). I'm not sure if there is an easy
> way to get a backtrace on every call to chdir() in every thread. I'm
> sure somebody more clever than me could figure out how to make gdb do it
> automatically, but it might be workable manually. I think the chdir was
> in the main thread.

Ah, that turned out to be quite easy. The culprit is probably:

(gdb) bt
#0  chdir () at ../sysdeps/unix/syscall-template.S:84
#1  0x00005555555fe259 in real_path_internal (path=0x5555559f6b30 "su:b/../.git/modules/su:b", die_on_error=1)
    at abspath.c:84
#2  0x00005555555fe48a in real_path (path=0x5555559f6b30 "su:b/../.git/modules/su:b") at abspath.c:135
#3  0x00005555556d09e6 in read_gitfile_gently (path=0x5555559f6ac0 "su:b/.git", return_error_code=0x0)
    at setup.c:555
#4  0x00005555556d19cf in resolve_gitdir (suspect=0x5555559f6ac0 "su:b/.git") at setup.c:1021
#5  0x00005555556e7e34 in is_submodule_populated (path=0x5555559f5ec8 "su:b") at submodule.c:244
#6  0x00005555555a0f05 in grep_submodule (opt=0x7fffffffd8b0, sha1=0x0, filename=0x5555559f5ec8 "su:b", 
    path=0x5555559f5ec8 "su:b") at builtin/grep.c:619
#7  0x00005555555a12ac in grep_cache (opt=0x7fffffffd8b0, pathspec=0x7fffffffd880, cached=0) at builtin/grep.c:700
#8  0x00005555555a36cb in cmd_grep (argc=0, argv=0x7fffffffdf40, prefix=0x0) at builtin/grep.c:1257
#9  0x000055555556603b in run_builtin (p=0x5555559b3ad8 <commands+984>, argc=4, argv=0x7fffffffdf40) at git.c:373
#10 0x00005555555662bc in handle_builtin (argc=4, argv=0x7fffffffdf40) at git.c:572
#11 0x000055555556641a in run_argv (argcp=0x7fffffffddfc, argv=0x7fffffffddf0) at git.c:630
#12 0x00005555555665a8 in cmd_main (argc=4, argv=0x7fffffffdf40) at git.c:702
#13 0x00005555555fde47 in main (argc=7, argv=0x7fffffffdf28) at common-main.c:40

So is_submodule_populated() needs to take a lock. But what's really
gross is that the _other_ threads need to lock just to call lstat().
Presumably it could be done as a reader/writer type of lock where many
"reader" threads can take the "I need to lstat()" lock simultaneously,
but block when an "I'm going to chdir()" writer holds it.

-Peff