RE: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

From: Suresh Thiagarajan
Date: Tue Dec 24 2013 - 04:14:03 EST

On Tue, Dec 24, 2013 at 1:59 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> On 12/23, Ingo Molnar wrote:
>> >
>> > * Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> >
>> > > Initially I thought that this is obviously wrong, irqsave/irqrestore
>> > > assume that "flags" is owned by the caller, not by the lock. And
>> > > iirc this was certainly wrong in the past.
>> > >
>> > > But when I look at spinlock.c it seems that this code can actually
>> > > work. _irqsave() writes to FLAGS after it takes the lock, and
>> > > _irqrestore() has a copy of FLAGS before it drops this lock.
>> >
>> > I don't think that's true: if it was then the lock would not be
>> > irqsave, a hardware-irq could come in after the lock has been taken
>> > and before flags are saved+disabled.
>> I do agree that this pattern is not safe, that is why I decided to ask.
>> But, unless I missed something, with the current implementation
>> spin_lock_irqsave(lock, global_flags) does:
>> unsigned long local_flags;
>> local_irq_save(local_flags);
>> spin_lock(lock);
>> global_flags = local_flags;
>> so the access to global_flags is actually serialized by lock.

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);

Here for unlocking, I could even use spin_unlock_irqrestore(&t->lock, t->lock_flags) directly instead of my_unlock() since t->lock_flags is updated only in my_lock and so there is no need to explicitly clear t->lock_flags. Please let me know if I miss anything here in serializing the global lock flag.

> You are right, today that's true technically because IIRC due to Sparc
> quirks we happen to return 'flags' as a return value - still it's very
> ugly and it could break anytime if we decide to do more aggressive
> optimizations and actually directly save into 'flags'.
> Note that even today there's a narrow exception: on UP we happen to
> build it the other way around, so that we do:
> local_irq_save(global_flags);
> __acquire(lock);
> This does not matter for any real code because on UP there is no
> physical lock and __acquire() is empty code-wise, but any compiler
> driven locking analysis tool using __attribute__ __context__(), if
> built on UP, would see the unsafe locking pattern.
> Thanks,
> Ingo
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