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From: Paulo Alcantara <>
To: Thomas Werschlein <>,
Subject: Re: Kernel lockup upon dfs_cache_update
Date: Tue, 10 Aug 2021 12:02:21 -0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Thomas Werschlein <> writes:

> Hi
> We are mounting SMB shares from Windows and FreeBSD servers to AD-joined (Ubuntu) Linux clients, using DFS and Kerberos.
> A sample /etc/fstab entry looks like this:
> // /files/shared cifs defaults,sec=krb5i,vers=3,seal,multiuser,noserverino,username=LINUXCLIENT$ 0 0
> The machine account LINUXCLIENT$ is able to browse the DFS tree and mount the shares.
> The credentials for the principal LINUXCLIENT$@D.EXAMPLE.COM are stored in the default keytab (/etc/krb5.keytab).
> In order to access a share a user needs a personal KRB ticket (either through kinit or GSSAPI delegated credentials upon ssh login).
> This works fine on Ubuntu 18.04 (4.15.0-153-generic Kernel, 2.10 cifs.ko), but doesn't on Ubuntu 20.04 (5.4.0-80-generic, 2.23).
> To be precise: it works for the first 10 hours (the ticket lifetime) even on Ubuntu 20.04.
> My findings so far:
> - this lockup (after 10h) *only* occurs when DFS is involved. Mounting a target share directly "survives" the ticket expiry
> - this problem does *not* manifest with the newest kernel/cifs.ko combo (5.14.0-051400rc5-generic/2.33)
> Therefore:
> Could it be, that the DFS patches contributed by Paulo Alcantara on June 4 2021 (
> fixed this problem?

Yes, that is possible.  We found out some deadlock and race scenarios
but couldn't remember if the above was one of them.  Still worth having
those commits backported.

> Two of his comments point into that direction: 
>   [...]
>   - keep SMB sessions alive as long as dfs mounts are actives in order
>     to refresh cached entries by using IPC tcons.
>   [...]
>   - skip unnecessary tree disconnect of IPCs when shutting down SMB
>     sessions (it didn't even work before).
> Am I on the right track here? And if so: are there any plans to backport Paulo's patches to current kernels?

Yes.  I don't know if there any plans, but the only tricky part is
changing the commits to work with the old mount api on <5.11 kernels.

  reply	other threads:[~2021-08-10 15:02 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-10 13:56 Thomas Werschlein
2021-08-10 15:02 ` Paulo Alcantara [this message]
2021-08-27 12:37   ` Thomas Werschlein

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