From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "zhengbin (A)" <zhengbin13@huawei.com>, Jan Kara <jack@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
"zhangyi (F)" <yi.zhang@huawei.com>,
renxudong1@huawei.com, Hou Tao <houtao1@huawei.com>
Subject: Re: [PATCH] Re: Possible FS race condition between iterate_dir and d_alloc_parallel
Date: Sun, 22 Sep 2019 22:29:34 +0100 [thread overview]
Message-ID: <20190922212934.GC29065@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=whpKgNTxjrenAed2sNkegrpCCPkV77_pWKbqo+c7apCOw@mail.gmail.com>
On Sat, Sep 14, 2019 at 09:49:21AM -0700, Linus Torvalds wrote:
> On Sat, Sep 14, 2019 at 9:16 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > OK, folks, could you try the following? It survives the local beating
> > so far.
>
> This looks like the right solution to me. Keep the locking simple,
> take the dentry refcount as long as we keep a ref to it in "*res".
*grumble*
Example of subtleties in the whole mess: this is safe for mainline
now, but only due to "devpts_pty_kill(): don't bother with d_delete()"
already merged. Without that, we are risking the following fun:
scan_positive() on devpts: finds a dentry, sees it positive, decides
to grab the sucker. Refcount is currently 1 (will become 2 after
we grab the reference).
devpts_pty_kill(): d_delete(dentry); on that sucker. Refcount is
currently (still) 1, so we simply make it negative.
scan_positive(): grabs an extra reference to now negative dentry.
devpts_pty_kill(): dput() drops refcount to 1 (what if it got there
before scan_positive() grabbed a reference? Nothing, really, since
scan_positive() is holding parent's ->d_lock; dput() wouldn't
have progressed through dentry_kill() until it managed to get
that, and it would've rechecked the refcount. So that's not
a problem)
scan_positive(): returns a reference to negative dentry to
dcache_readdir(). Which proceeds to oops on
if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
d_inode(next)->i_ino, dt_type(d_inode(next))))
since d_inode(next) is NULL.
With the aforementioned commit it *is* safe, since the dentry remains
positive (and unhashed), so we simply act as if dcache_readdir() has
won the race and emitted a record for the sucker killed by devpts_pty_kill().
IOW, backports will either need to bring that commit in as well, or
they'll need to play silly buggers along the lines of
if (simple_positive(d) && !--count) {
spin_lock(&d->d_lock);
if (likely(simple_positive(d)))
found = dget_dlock(d);
spin_unlock(&d->d_lock);
if (found)
break;
count = 1; // it's gone, keep scanning
}
Probably the latter, since it's less dependent on some other place
doing what devpts used to do...
next prev parent reply other threads:[~2019-09-22 22:30 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 14:44 Possible FS race condition between iterate_dir and d_alloc_parallel zhengbin (A)
2019-09-03 15:40 ` Al Viro
2019-09-03 15:41 ` Al Viro
2019-09-04 6:15 ` zhengbin (A)
2019-09-05 17:47 ` Al Viro
2019-09-06 0:55 ` Jun Li
2019-09-06 2:00 ` Al Viro
2019-09-06 2:32 ` zhengbin (A)
2019-09-09 14:10 ` zhengbin (A)
2019-09-09 14:59 ` Al Viro
2019-09-09 15:10 ` zhengbin (A)
[not found] ` <7e32cda5-dc89-719d-9651-cf2bd06ae728@huawei.com>
2019-09-10 21:53 ` Al Viro
2019-09-10 22:17 ` Al Viro
2019-09-14 16:16 ` [PATCH] " Al Viro
2019-09-14 16:49 ` Linus Torvalds
2019-09-14 17:01 ` Al Viro
2019-09-14 17:15 ` Linus Torvalds
2019-09-14 20:04 ` Al Viro
2019-09-14 22:57 ` Linus Torvalds
2019-09-15 0:50 ` Al Viro
2019-09-15 1:41 ` Linus Torvalds
2019-09-15 16:02 ` Al Viro
2019-09-15 17:58 ` Linus Torvalds
2019-09-21 14:07 ` Al Viro
2019-09-21 16:21 ` Linus Torvalds
2019-09-21 17:18 ` Al Viro
2019-09-21 17:38 ` Linus Torvalds
2019-09-24 2:52 ` Al Viro
2019-09-24 13:30 ` Josef Bacik
2019-09-24 14:51 ` Al Viro
2019-09-24 15:01 ` Josef Bacik
2019-09-24 15:11 ` Al Viro
2019-09-24 15:26 ` Josef Bacik
2019-09-24 16:33 ` Al Viro
[not found] ` <CAHk-=wiJ1eY7y6r_cFNRPCqD+BJZS7eJeQFO6OrXxRFjDAipsQ@mail.gmail.com>
2019-09-29 5:29 ` Al Viro
2019-09-25 11:59 ` Amir Goldstein
2019-09-25 12:22 ` Al Viro
2019-09-25 12:34 ` Amir Goldstein
2019-09-22 21:29 ` Al Viro [this message]
2019-09-23 3:32 ` zhengbin (A)
2019-09-23 5:08 ` Al Viro
[not found] ` <20190916020434.tutzwipgs4f6o3di@inn2.lkp.intel.com>
2019-09-16 2:58 ` 266a9a8b41: WARNING:possible_recursive_locking_detected Al Viro
2019-09-16 3:03 ` Al Viro
2019-09-16 3:44 ` Linus Torvalds
2019-09-16 17:16 ` Al Viro
2019-09-16 17:29 ` Al Viro
[not found] ` <bd707e64-9650-e9ed-a820-e2cabd02eaf8@huawei.com>
2019-09-17 12:01 ` Al Viro
2019-09-19 3:36 ` zhengbin (A)
2019-09-19 3:55 ` Al Viro
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