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VM66545: ABENDSZC001 DURING EDEVICE CSCH PROCESSING

A fix is available

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APAR status

  • Closed as program error.

Error description

  • In rare cases, Linux error recovery for I/O to EDEVICEs will
    lead to one I/O being placed on the deferred work queue twice,
    leading to an SZC001.
    
    When Linux is attempting to recover from failed I/O to EDEVICEs,
    it will issue a clear subchannel (CSCH) to the EDEVICE in order
    to clear the problem I/O.  When the z/VM SCSI driver attempts to
    purge the I/O as requested, the I/O gets placed on the deferred
    work queue multiple times.  When it comes time to perform
    completion tasks for the deferred work queue, we end up in what
    is essentially an infinite loop calling iodone() for the same
    I/O over and over.
    The first call results in a SZC001 soft abend. While that soft
    abend processing is occurring, the second call results in a
    second SZC001 which gets translated into a hard abend, bringing
    down the sytem.
    

Local fix

  • N/A
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All users of z/VM EDEVICEs                   *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    ****************************************************************
    * RECOMMENDATION: APPLY PTF                                    *
    ****************************************************************
    In rare cases, when linux attempts to recover from I/O errors on
    z/VM EDEVICEs, z/VM will abend with an SZC001.
    
    The problem stems from deferred work queue processing that
    occurs in the z/VM SCSI driver during clear subchannel
    processing.  In some instances, an element may be placed on
    the deferred work queue multiple times which leads z/VM
    to attempt to perform I/O completion processing on the same
    I/O multiple times.  The first time, things work as expected.
    The second time we encounter an SZC001 (soft) abend.  The
    third time we encounter another SZC001 abend which gets
    converted to a hard abend because the first soft abend has not
    yet completed processing.
    

Problem conclusion

  • Code has been added to ensure an element does not already exist
    on the deferred work queue before adding it to the queue.
    

Temporary fix

Comments

APAR Information

  • APAR number

    VM66545

  • Reported component name

    VM CP CP

  • Reported component ID

    568411202

  • Reported release

    710

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2021-06-18

  • Closed date

    2021-07-29

  • Last modified date

    2022-12-13

  • APAR is sysrouted FROM one or more of the following:

  • APAR is sysrouted TO one or more of the following:

    UM35889 UM35890

Modules/Macros

  • HCWPA2
    

Fix information

  • Fixed component name

    VM CP CP

  • Fixed component ID

    568411202

Applicable component levels

  • R710 PSY UM35889

       UP21/08/12 P 2201

  • R720 PSY UM35890

       UP21/09/30 P 2102

Fix is available

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Document Information

Modified date:
13 December 2022