linux-kernel - Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

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Message-ID: <CAA9_cme0MZvz_6VXXpP3ATkR6p_OznHpVtyyNxA+HK2iBvb7Rg@mail.gmail.com>
Date:	Fri, 3 Jan 2014 12:02:06 -0800
From:	Dan Williams <dan.j.williams@...il.com>
To:	Suresh Thiagarajan <Suresh.Thiagarajan@...s.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Jason Seba <jason.seba42@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tomas Henzl <thenzl@...hat.com>, Jack Wang <xjtuwjp@...il.com>,
	Viswas G <Viswas.G@...s.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"JBottomley@...allels.com" <JBottomley@...allels.com>,
	Vasanthalakshmi Tharmarajan 
	<Vasanthalakshmi.Tharmarajan@...s.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

On Thu, Jan 2, 2014 at 2:31 AM, Suresh Thiagarajan
<Suresh.Thiagarajan@...s.com> wrote:
>
>
> On Fri, Dec 27, 2013 at 9:48 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 12/24, Suresh Thiagarajan wrote:
>>>
>>> Below is a small pseudo code on protecting/serializing the flag for global access.
>>> struct temp
>>> {
>>>       ...
>>>       spinlock_t lock;
>>>       unsigned long lock_flags;
>>> };
>>> void my_lock(struct temp *t)
>>> {
>>>                unsigned long flag; // thread-private variable as suggested
>>>                spin_lock_irqsave(&t->lock, flag);
>>>                t->lock_flags = flag; //updating inside critical section now to serialize the access to flag
>>> }
>>>
>>> void my_unlock(struct temp *t)
>>> {
>>>                unsigned long flag = t->lock_flags;
>>>                t->lock_flags = 0;  //clearing it before getting out of critical section
>>>                spin_unlock_irqrestore(&t->lock, flag);
>>> }
>>
>> Yes, this should work as a quick fix. And you do not need to clear ->lock_flags
>> in my_unlock().
>>
>> But when I look at original patch again, I no longer understand why do
>> you need pm8001_ha->lock_flags at all. Of course I do not understand this
>> code, I am sure I missed something, but at first glance it seems that only
>> this sequence
>>
>>         spin_unlock_irq(&pm8001_ha->lock);
>>         t->task_done(t);
>>         spin_lock_irq(&pm8001_ha->lock);
>>
>> should be fixed?
>>
>> If yes, why you can't simply do spin_unlock() + spin_lock() around
>> t->task_done() ? This won't enable irqs, but spin_unlock_irqrestore()
>> doesn't necessarily enables irqs too, so ->task_done() can run with
>> irqs disabled?
>>
>> And note that the pattern above has a lot of users, perhaps it makes
>> sense to start with something like the patch below?
>
> Thanks James, Oleg and all for your inputs.
> Will start with review and testing this patch and then work/investigate to keep shortest and clearest critical
> section so that we can have the lock and unlock within the same routine.
>

Fwiw we solved this in libsas a while back with a similar pattern
proposed by Oleg:

unsigned long flags;

local_irq_save(flags);
spin_unlock(lock);
...
spin_lock_lock(lock);
local_irq_restore(flags);

See commit 312d3e56119a "[SCSI] libsas: remove ata_port.lock
management duties from lldds"

--
Dan
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