Messages in this thread | | | Subject | Re: [PATCH] exit: fix a race in release_task when flushing the dentry | From | Wen Yang <> | Date | Sun, 29 Nov 2020 22:43:30 +0800 |
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在 2020/11/29 下午2:05, Greg Kroah-Hartman 写道: > On Sat, Nov 28, 2020 at 11:28:53PM +0800, Wen Yang wrote: >> >> >> 在 2020/11/28 下午10:05, Greg Kroah-Hartman 写道: >>> On Sat, Nov 28, 2020 at 09:59:09PM +0800, Wen Yang wrote: >>>> >>>> >>>> 在 2020/11/28 下午4:06, Greg Kroah-Hartman 写道: >>>>> On Sat, Nov 28, 2020 at 02:47:22PM +0800, Wen Yang wrote: >>>>>> [ Upstream commit 7bc3e6e55acf065500a24621f3b313e7e5998acf ] >>>>> >>>>> No, that is not this commit at all. >>>>> >>>>> What are you wanting to have happen here? >>>>> >>>>> confused, >>>>> >>>>> greg k-h >>>>> >>>> >>>> Thanks. >>>> Let's explain it briefly: >>>> >>>> The dentries such as /proc/<pid>/ns/ipc have the DCACHE_OP_DELETE flag, they >>>> should be deleted when the process exits. >>>> Suppose the following race appears: >>>> >>>> release_task dput >>>> -> proc_flush_task >>>> -> dentry->d_op->d_delete(dentry) >>>> -> __exit_signal >>>> -> dentry->d_lockref.count-- and return. >>>> >>>> >>>> In the proc_flush_task function, because another processe is using this >>>> dentry, it cannot be deleted; >>>> In the dput function, d_delete may be executed before __exit_signal (the pid >>>> has not been unhashed), so that d_delete returns false and the dentry can >>>> not be deleted. >>>> >>>> So this dentry is still caches (count is 0), and its parent dentries are >>>> also caches, and those dentries can only be deleted when drop_caches is >>>> manually triggered. >>>> >>>> >>>> In the release_task function, we should move proc_flush_task after the >>>> tasklist_lock is released(Just like the commit >>>> 7bc3e6e55acf065500a24621f3b313e7e5998acf did). >>> >>> I do not understand, is this a patch being submitted for the main kernel >>> tree, or for a stable kernel release? >>> >>> If stable, please read: >>> https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html >>> for how to do this properly. >>> >>> If main kernel tree, you can't have the "Upstream commit" line in the >>> changelog text as that makes no sense at all. >> >> >> Hi, >> This patch is submitted to the stable branches (from 4.9.y >> to 5.6.y). >> >> This problem can also be solved if the following patch could be ported to >> the stable branch: >> 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc") >> 26dbc60f385f ("proc: Generalize proc_sys_prune_dcache into >> proc_prune_siblings_dcache") >> f90f3cafe8d5 ("proc: Use d_invalidate in proc_prune_siblings_dcache") >> >> However, the above-mentioned patches modify too much code (more than 100 >> lines), and there may also be some undiscovered bugs. >> >> So the safer method may be to apply this small patch(also ported from the >> equivalent fix already exist in Linus’ tree). >> >> We will reformat the patch later. > > We always prefer to take the original, upstream patches, instead of > one-off changes as almost always, those one-off changes end up being > wrong and hard to work with over time. > > So if we need more than one patch to solve this reported problem, that's > fine, can you test the above series of patches and provide a backported > set of them that we can use for this? >
Ok, we will follow your suggestions. Thanks.
-- Best wishes, Wen
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