[PATCH v2 09/30] cxlflash: Fix to stop interrupt processing on remove

[Matthew R. Ochs]

> On Sep 18, 2015, at 6:59 AM, Tomas Henzl <thenzl at redhat.com> wrote:
> On 17.9.2015 19:16, Matthew R. Ochs wrote:
>>> On Sep 17, 2015, at 7:38 AM, Tomas Henzl <thenzl at redhat.com> wrote:
>>> 
>>> On 16.9.2015 18:53, Matthew R. Ochs wrote:
>>>> Interrupt processing can run in parallel to a remove operation. This
>>>> can lead to a condition where the interrupt handler is processing with
>>>> memory that has been freed.
>>>> 
>>>> To avoid processing an interrupt while memory may be yanked, check for
>>>> removal while in the interrupt handler. Bail when removal is imminent.
>>>> 
>>>> Signed-off-by: Matthew R. Ochs <mrochs at linux.vnet.ibm.com>
>>>> Signed-off-by: Manoj N. Kumar <manoj at linux.vnet.ibm.com>
>>>> ---
>>>> drivers/scsi/cxlflash/common.h |  2 ++
>>>> drivers/scsi/cxlflash/main.c   | 21 +++++++++++++++------
>>>> 2 files changed, 17 insertions(+), 6 deletions(-)
>>>> 
>>>> diff --git a/drivers/scsi/cxlflash/common.h b/drivers/scsi/cxlflash/common.h
>>>> index 1abe4e0..03d2cc6 100644
>>>> --- a/drivers/scsi/cxlflash/common.h
>>>> +++ b/drivers/scsi/cxlflash/common.h
>>>> @@ -103,6 +103,8 @@ struct cxlflash_cfg {
>>>> 	enum cxlflash_lr_state lr_state;
>>>> 	int lr_port;
>>>> 
>>>> +	atomic_t remove_active;
>>>> +
>>>> 	struct cxl_afu *cxl_afu;
>>>> 
>>>> 	struct pci_pool *cxlflash_cmd_pool;
>>>> diff --git a/drivers/scsi/cxlflash/main.c b/drivers/scsi/cxlflash/main.c
>>>> index 6e85c77..89ee648 100644
>>>> --- a/drivers/scsi/cxlflash/main.c
>>>> +++ b/drivers/scsi/cxlflash/main.c
>>>> @@ -892,6 +892,7 @@ static void cxlflash_remove(struct pci_dev *pdev)
>>>> 	spin_unlock_irqrestore(&cfg->tmf_waitq.lock, lock_flags);
>>>> 
>>>> 	cfg->state = STATE_FAILTERM;
>>>> +	atomic_inc(&cfg->remove_active);
>>> Hi Matthew,
>>> you could just call term_afu at this point, this way you don't
>>> need an additional check in all irq functions.
>>> Cheers,
>>> Tomas
>> Hi Tomas,
>> 
>> We actually do call term_afu() a few lines down from here. I don't follow
>> how moving it here would help things.
> 
> When you disable ints sooner (that is what term_afu does ?) you'll get no
> more ints later isn't this what you want?

Correct, that's what we want.

>> The reason for the atomic was to provide something lightweight that we
>> could check _inside_ the processing loop for the read-response queue
>> handler. A check outside that loop doesn't really provide much in terms
>> of closing or narrowing down the window of when freed memory can be
>> accessed.
>> 
>> As David Laight correctly pointed out, this approach does not completely
>> close the window. We'd need something heavier to fully protect (e.g. a lock
>> to wrap around the entire loop). I will look into adding this in a future cycle
>> when I can adequately test.
> 
> term_afu calls free_irq and this function
> does not return until any executing interrupts for have completed.
> This is the sync mechanism you need, it's lightweight
> (does not add an additional check to your irq functions)
> and closes the race window completely.

Thanks for clarifying!

I looked at this closer and you are correct, free_irq() guarantees not
to return until the interrupt handler has completed. The current location
of term_afu() is appropriate as the memory that the handler touches is
not freed until the very end [by free_mem() and scsi_host_put()] of the
remove. Thus we can simply ignore this patch (I'll remove it in a v3).


-matt