[Gluster-devel] Why to fork so many times?

[Daniel]

On Tue, 2011-06-07 at 14:44 +0530, Pavan T C wrote:
> On Tuesday 07 June 2011 02:08 PM, Daniel wrote:
> > Thank you, Pavan. It clarifies most of my doubts.
> >
> > Only I can't find the branch of 'if sync-mtab == 1'. And I thought it
> > was without "YES fork()". Could you explain more about that, please?
> >
> 
> Oh, the variable mtab_pid is what indicates whether the sync-mtab option 
> is set or not. Trace back the caller of fuse_mnt_add_mount(). The call 
> stack looks like below;
> 
> fuse_bridge.c:init() -> gf_fuse_mount() -> fuse_mount_sys() -> 
> fuse_mnt_add_mount().
> 
> In gf_fuse_mount(), the last parameter is passed as :
> sync_mtab ? &ctx->mtab_pid : NULL

Is the ctx a _glusterfs_ctx ?

> So, if the "sync-mtab" option is set, mtab_pid will be non-NULL. 
> Otherwise it will be NULL.
> 
> Now you can relate back to the condition in fuse_mnt_add_mount().
> If mtab_pid is NULL, the "NO" condition in the flow-chart I provided 
> comes into picture. Otherwise the code follows the YES condition.
> 
> Pavan
> 
> > Thanks a lot.
> >
> > Dan
> >
> > On Tue, 2011-06-07 at 12:29 +0530, Pavan T C wrote:
> >> On Tuesday 07 June 2011 10:54 AM, Daniel wrote:
> >>> Hello hackers:
> >>>
> >>> I am learning glusterfs and get one question when I reading
> >>> contrib/fuse-lib/mount.c fuse_mnt_add_mount(). Why does it fork four
> >>> times and the grand-grand-child exec("/bin/mount")? What is the benefit
> >>> and the consideration?
> >>
> >> As I see it, it forks thrice. Here is what I understand from the code:
> >>
> >> +---------------------+
> >> | fuse_mnt_add_mount()|<------------------------------+
> >> | fork and wait for   |                               |
> >> | child to return     |                               |
> >> +-----+---------------+                               |
> >>         |                                               |
> >>         |                                               |
> >>         X                                               |
> >>        / \                                              |
> >>       /if \                                             |
> >>      /sync-\    NO    +---------------------------+     |
> >>     X mtab  X-------->| Do async ops by returning |     |
> >>      \ == 1/   fork() | from here so that parent  |     |
> >>       \   /           | does not wait. But before |     |
> >>        \ /            | doing that, create a dummy|     |
> >>         X             | parent to wait for child  |     |
> >>         | YES         | that does the /bin/mount  |     |
> >>         |             +------------+--------------+     |
> >>         | fork()                   |                    |
> >>         V                          |                    |
> >> +-----+----------+               | fork() ->  Parent returns here.
> >> | Child: execl   |               v
> >> | /bin/mount with|         +---------------+
> >> | -f option to   |         |dummy parent:  |
> >> | only populate  |  fork() | fork and wait |
> >> | mtab but don't |<--------+ for /bin/mount|
> >> | actually mount |         | to return     |
> >> | anything.      |         +---------------+
> >> +----------------+
> >>
> >> HTH,
> >> Pavan
> >>
> >>>
> >>> Thank you
> >>>
> >>> Daniel
> >>>
> >>>
> >>> _______________________________________________
> >>> Gluster-devel mailing list
> >>> Gluster-devel at nongnu.org
> >>> https://lists.nongnu.org/mailman/listinfo/gluster-devel
> >>
> >
> >
>