On 12/23, Oleg Nesterov wrote:
>
> Perhaps we should ask the maintainers upstream? Even if this works, I am
> not sure this is _supposed_ to work. I mean, in theory spin_lock_irqave()
> can be changed as, say
>
> #define spin_lock_irqsave(lock, flags) \
> do { \
> local_irq_save(flags); \
> spin_lock(lock); \
> } while (0)
>
> (and iirc it was defined this way a long ago). In this case "flags" is
> obviously not protected.
Yes, lets ask the maintainers.
In short, is this code
spinlock_t LOCK;
unsigned long FLAGS;
void my_lock(void)
{
spin_lock_irqsave(&LOCK, FLAGS);
}
void my_unlock(void)
{
spin_unlock_irqrestore(&LOCK, FLAGS);
}
correct or not?
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.
And it turns out, some users assume this should work, for example
arch/arm/mach-omap2/powerdomain.c:
pwrdm_lock() and pwrdm_unlock()
drivers/net/wireless/brcm80211/brcmfmac/fwsignal.c:
brcmf_fws_lock() and brcmf_fws_unlock()
seem to do exactly this. Plus the pending patch for drivers/scsi/pm8001/.
So is it documented somewhere that this sequence is correct, or the code
above should be changed even if it happens to work?
Oleg.
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