From: Liangyan <liangyan.peng@linux.alibaba.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>,
Miklos Szeredi <miklos@szeredi.hu>,
linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] ovl: fix dentry leak in ovl_get_redirect
Date: Tue, 22 Dec 2020 02:15:46 +0800 [thread overview]
Message-ID: <f3e1d7bc-b350-6fe6-7a26-7c65f7122023@linux.alibaba.com> (raw)
In-Reply-To: <20201221173538.GQ3579531@ZenIV.linux.org.uk>
Exactly, i missed this definition of d_lock and treat it as a single
member in dentry.
#define d_lock d_lockref.lock
Thanks for the explanation. i will post a new patch as your suggestion.
Regards,
Liangyan
On 20/12/22 上午1:35, Al Viro wrote:
> On Tue, Dec 22, 2020 at 12:51:27AM +0800, Liangyan wrote:
>> This is the race scenario based on call trace we captured which cause the
>> dentry leak.
>>
>>
>> CPU 0 CPU 1
>> ovl_set_redirect lookup_fast
>> ovl_get_redirect __d_lookup
>> dget_dlock
>> //no lock protection here spin_lock(&dentry->d_lock)
>> dentry->d_lockref.count++ dentry->d_lockref.count++
>>
>>
>> If we use dget_parent instead, we may have this race.
>>
>>
>> CPU 0 CPU 1
>> ovl_set_redirect lookup_fast
>> ovl_get_redirect __d_lookup
>> dget_parent
>> raw_seqcount_begin(&dentry->d_seq) spin_lock(&dentry->d_lock)
>> lockref_get_not_zero(&ret->d_lockref) dentry->d_lockref.count++
>
> And?
>
> lockref_get_not_zero() will observe ->d_lock held and fall back to
> taking it.
>
> The whole point of lockref is that counter and spinlock are next to each
> other. Fastpath in lockref_get_not_zero is cmpxchg on both, and
> it is taken only if ->d_lock is *NOT* locked. And the slow path
> there will do spin_lock() around the manipulations of ->count.
>
> Note that ->d_lock is simply ->d_lockref.lock; ->d_seq has nothing
> to do with the whole thing.
>
> The race in mainline is real; if you can observe anything of that
> sort with dget_parent(), we have much worse problem. Consider
> dget() vs. lookup_fast() - no overlayfs weirdness in sight and the
> same kind of concurrent access.
>
> Again, lockref primitives can be safely mixed with other threads
> doing operations on ->count while holding ->lock.
>
prev parent reply other threads:[~2020-12-21 18:16 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-20 12:09 [PATCH v2] ovl: fix dentry leak in ovl_get_redirect Liangyan
2020-12-21 6:14 ` Liangyan
2020-12-21 6:26 ` Al Viro
2020-12-21 11:14 ` Joseph Qi
2020-12-21 12:11 ` Al Viro
2020-12-21 16:51 ` Liangyan
2020-12-21 17:35 ` Al Viro
2020-12-21 18:15 ` Liangyan [this message]
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