Hi Ingo,
in pre 2.6.33-rc1 commit 979f693d you wrote: "I'd like to use
printk_ratelimit() in atomic context, but that's not possible right now
due to the spinlock usage this commit introduced more than a year ago:
717115e: printk ratelimiting rewrite".?,?
By not possible, do you mean it would be an outright bug to call
printk_ratelimit in atomic context prior to 979f693d, or merely a
performance issue? If the former, under which circumstances would the
bug hit?
I'm asking because one of my commits actually introduced a
printk_ratelimit use in an interrupt handler shortly before 2.6.32 was
released. In my testing, it didn't occur to me that there might be a
problem.
?http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=979f693def9084a452846365dfde5dcb28366333
?http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=717115e1a5856b57af0f71e1df7149108294fc10
--
Stefan Richter
-=====-==--= ==-- =-=--
http://arcgraph.de/sr/
* Stefan Richter <[email protected]> wrote:
> Hi Ingo,
>
> in pre 2.6.33-rc1 commit 979f693d you wrote: "I'd like to use
> printk_ratelimit() in atomic context, but that's not possible right now
> due to the spinlock usage this commit introduced more than a year ago:
> 717115e: printk ratelimiting rewrite".?,?
>
> By not possible, do you mean it would be an outright bug to call
> printk_ratelimit in atomic context prior to 979f693d, or merely a
> performance issue? If the former, under which circumstances would the
> bug hit?
>
> I'm asking because one of my commits actually introduced a
> printk_ratelimit use in an interrupt handler shortly before 2.6.32 was
> released. In my testing, it didn't occur to me that there might be a
> problem.
>
> ?http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=979f693def9084a452846365dfde5dcb28366333
> ?http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=717115e1a5856b57af0f71e1df7149108294fc10
There used to be a global lock:
static DEFINE_SPINLOCK(ratelimit_lock);
which is now per ratelimit context. The case i was after was to use the
ratelimit state from NMI context. With NMIs we can lock up if an NMI hits when
some other code uses the ratelimit code. It's a small but existing race
window.
OTOH, IRQ context use of printk ratelimit was safe before (and after) this
commit - so i think your code should be safe too.
Hope that helps,
Ingo
Ingo Molnar wrote:
> There used to be a global lock:
>
> static DEFINE_SPINLOCK(ratelimit_lock);
>
> which is now per ratelimit context. The case i was after was to use the
> ratelimit state from NMI context. With NMIs we can lock up if an NMI hits when
> some other code uses the ratelimit code. It's a small but existing race
> window.
Ah, I see.
> OTOH, IRQ context use of printk ratelimit was safe before (and after) this
> commit - so i think your code should be safe too.
Thanks for the explanation; that's reassuring.
--
Stefan Richter
-=====-==--= ==-- ===-=
http://arcgraph.de/sr/