linux-kernel - Re: [PATCH V2] scsi: libsas: Directly kick-off EH when ATA device fell off

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Message-ID: <257dcd6c-2ffd-f518-9b13-c309348088d9@oracle.com>
Date:   Mon, 19 Dec 2022 15:55:01 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Jason Yan <yanaijie@...wei.com>,
        Xingui Yang <yangxingui@...wei.com>, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, damien.lemoal@...nsource.wdc.com,
        linux-ide@...r.kernel.org, hare@...e.com, hch@....de
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, prime.zeng@...ilicon.com,
        kangfenglong@...wei.com
Subject: Re: [PATCH V2] scsi: libsas: Directly kick-off EH when ATA device
 fell off

On 19/12/2022 15:28, Jason Yan wrote:
>>> +    if (test_bit(SAS_DEV_GONE, &dev->state) && dev_is_sata(dev))
>>> +        sas_ata_device_link_abort(dev, false);
>>
>> Firstly, I think that there is a bug in sas_ata_device_link_abort() -> 
>> ata_link_abort() code in that the host lock in not grabbed, as the 
>> comment in ata_port_abort() mentions. Having said that, libsas had 
>> already some dodgy host locking usage - specifically dropping the lock 
>> for the queuing path (that's something else to be fixed up ... I think 
> 
> Taking big locks in queuing path is not a good idea. This will bring 
> down performance.

But it is expected that ata_qc_issue() should be called with that the 
host lock grabbed (and keep it).

I think that the reason libsas drops the lock is because some LLDD 
queuecommand CBs calls task_done() in some error paths. If we kept the 
lock held, then we could have a deadlock, for example:

sas_ata_qc_issue (has lock) -> lldd_execute_task() = 
pm8001_queue_command() -> task_done() = sas_ata_task_done() -> grab host 
lock => deadlock.

Thanks,
John