CIFS hangs in smb2_find_smb_tcon()

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Environment

SUSE Linux Enterprise Server 15 SP5

SUSE Linux Enterprise Server 15 SP6

Situation

CIFS access may hang which leads to "soft lockup" errors and high CPU utilization in the system.

If a kdump is generated when the CIFS is hanging, the "cifs" module is seen hanging with the following stack trace:

>  #7 [ffffb91ac0a93d98] native_queued_spin_lock_slowpath at ffffffff93e807b1
>  #8 [ffffb91ac0a93d98] _raw_spin_lock at ffffffff93e80445
>  #9 [ffffb91ac0a93da0] __cifs_put_smb_ses at ffffffffc0bf5ea0 [cifs]
> #10 [ffffb91ac0a93dd0] smb2_find_smb_tcon at ffffffffc0c3474e [cifs]
> #11 [ffffb91ac0a93df0] smb2_handle_cancelled_mid at ffffffffc0c37142 [cifs]
> #12 [ffffb91ac0a93e10] release_mid at ffffffffc0c17eb8 [cifs]
> #13 [ffffb91ac0a93e50] cifs_demultiplex_thread at ffffffffc0bf496e [cifs]

Resolution

This bug is fixed in the following kernel versions:
SLE 15 SP5: 5.14.21-150500.55.73.1
SLE 15 SP6: 6.4.0-150600.23.17.1

Cause

A deadlock can happen in "smb2_find_smb_tcon()" where the function attempts to acquire an already acquired spinlock.

Disclaimer

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