Hello,
> Hello? This is Sungjong. Currently, I am unable to reply using my
> samsung.com email, so I am responding with my other Gmail account.
>
> On 2/28/25 14:44, Namjae Jeon wrote:
> > On Fri, Feb 28, 2025 at 7:48 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> There's a really odd comment in that thing:
> >> /*
> >> * Unhashed alias is able to exist because of revalidate()
> >> * called by lookup_fast. You can easily make this status
> >> * by calling create and lookup concurrently
> >> * In such case, we reuse an alias instead of new dentry
> >> */
> >> and AFAICS it had been there since the original merge. What I don't
> >> understand is how the hell could revalidate result in that -
> >> exfat_d_revalidate() always returns 1 on any positive dentry and alias
> is
> >> obviously positive (it has the same inode as the one we are about to
> use).
> >>
> >> It mentions a way to reproduce that, but I don't understand what does
> >> that refer to; could you give details?
I tested it on an arm64 device running on Android with kernel v5.15, v6.6.
And I can see unhashed-positive dentry with the following simple TC,
even if it is not an Android stacked fs environment.
* TestCase
- thread1: while(1) { mkdir(A) and rmdir(A) }
- thread2: while(1) { stat(A) }
This is due to the characteristics of exfat allowing negative dentry and
considering CI in d_revalidate. As mentioned in the comment,
unhashed-positive dentry can exist in a situation where mkdir
and stat are competing, and it can be dropped, but exfat_lookup has
been implemented to reuse(rehash) this dentry.
I hope the following callstack will help you understand.
Thank you.
<CPU 0> <CPU 1>
do_mkdirat
user_path_create
*filename_create
inode_lock(I_MUTEX_PARENT)
__lookup_hash(LOOKUP_CREATE | LOOKUP_EXCL)
dentry = d_alloc
exfat_lookup
lock(sbi->s_lock)
no_exist_file
unlock(sbi->s_lock)
d_version_set
d_splice_alias
__d_add() //hashed-negative dentry here
*vfs_mkdir
exfat_mkdir
lock(sbi->s_lock)
inc_iversion(dir)//iversion diff occured
vfs_statx
filename_lookup
path_lookupat
lookup_last
work_component
lookup_fast
dentry = __d_lookup
d_revalidate(dentry) //because of iversion diff
d_invalidate(dentry)
__d_drop(dentry)
lookup_slow
inode_lock_shared(dir)
inc_nlink(dir)
inode = build_inode
inc_iversion(inode)
d_instantiate(dentry, inode) //* unhashed-positive dentry here
unlock(sbi->s_lock)
*done_path_create
inode_unlock(I_MUTEX_PARENT)
__lookup_slow
exfat_lookup
lock(sbi->s_lock)
inode = exfat_build_inode
alias = d_find_alias(inode)
d_unhashed(alias)? // * found unhashed positive dentry *
d_rehash(alias)
...
