lkml.org 
[lkml]   [2003]   [Dec]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [DOCUMENTATION] Revised Unreliable Kernel Locking Guide
Hi Rusty,

From Chapter 4.1:

> |spin_lock_irqsave()| (include/linux/spinlock.h) is a variant which
> saves whether interrupts were on or off in a flags word, which is
> passed to |spin_unlock_irqrestore()|. This means that the same code
> can be used inside an hard irq handler (where interrupts are already
> off) and in softirqs (where the irq disabling is required).

Interrupts are typically on within the hard irq handler.
spin_lock() is usually ok because an interrupt handler is never
reentered. Thus if a lock is only accessed from a single irq handler,
then spin_lock() is the faster approach. That's why many nic drivers use
spin_lock instead of spin_lock_irqsave() in their irq handlers.
OTHO: if a driver lock is a global resource that is used from different
irqs, then it must either use _irqsave(), or set SA_INTERRUPT.
Examples: rtc_lock relies on SA_INTERRUPT: it's touched from the rtc irq
and the timer irq path, and both rtc and timer set SA_INTERRUPT.
I assume ide relies on _irqsave(), but the code is too difficult to follow.

Btw, perhaps you could add the 2nd synchronization approach for
interrupts: if it's an extremely rare event, then no lock at all in the
irq handler (no reentrancy guaranteed by the kernel), and
spin_lock+disable_irq() in the softirq/tasklet. My network drivers use
that to synchronize packet rx with close.

--
Manfred

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:59    [W:0.022 / U:1.116 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site