From: William Lee Irwin III <wli@holomorphy.com>
To: Robert Love <rml@tech9.net>
Cc: John Levon <movement@marcelothewonderpenguin.com>,
linux-kernel@vger.kernel.org, akpm@digeo.com
Subject: Re: Sleeping function called from illegal context...
Date: Sat, 28 Sep 2002 17:50:19 -0700 [thread overview]
Message-ID: <20020929005019.GD22942@holomorphy.com> (raw)
In-Reply-To: <1033237664.22582.167.camel@phantasy>
On Sat, 2002-09-28 at 13:24, John Levon wrote:
>> NMI interrupt handler cannot block so it trylocks against a spinlock
>> instead. Buffer processing code needs to block against concurrent NMI
>> interrupts so takes the spinlock for them. All actual blocks on the
>> spinlock are beneath a down() on another semaphore, so a sleep whilst
>> holding the spinlock won't actually cause deadlock.
On Sat, Sep 28, 2002 at 02:27:44PM -0400, Robert Love wrote:
> If all accesses to the spinlock are taken under a semaphore, then the
> spinlock is not needed (i.e. the down'ed semaphore provides the same
> protection), or am I missing something?
> If this is not the case - e.g. there are other accesses to these locks -
> then you cannot sleep, no?
> I really can think of no case in which it is safe to sleep while holding
> a spinlock or otherwise atomic. If it is, then the atomicity is not
> needed, sort of by definition.
Actually, though he may be using a spinlock_t, when used this way, it
is not a spinlock, but rather a semaphore-like construct like PG_locked.
Spinlocks include blocking via busywait semantics, which this usage
does not have. It just happens to use the same data type. There are
other interesting abuses of spinlock-like constructs in "advanced"
locks, for instance, in non-sleeping handoff-scheduled queueing locks
(e.g. MCS spinlocks and rwlocks) it's a common idiom for one waiter to
set a "blocked" bit or lock word and then spin on it until another
waiter and/or cpu manipulating the lock clears it.
Bill
prev parent reply other threads:[~2002-09-29 0:47 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-09-27 23:30 Sleeping function called from illegal context Greg KH
2002-09-27 23:50 ` Andrew Morton
2002-09-28 0:43 ` (more) " Luc Van Oostenryck
2002-09-28 1:22 ` Andrew Morton
2002-09-28 0:44 ` Luc Van Oostenryck
2002-09-28 1:24 ` Andrew Morton
2002-09-28 2:15 ` Greg KH
2002-09-28 0:44 ` Luc Van Oostenryck
2002-09-28 1:27 ` Andrew Morton
2002-09-30 12:50 ` [Lksctp-developers] " Jon Grimm
2002-09-30 13:34 ` Jon Grimm
2002-09-28 2:04 ` Andre Hedrick
2002-09-28 3:06 ` Robert Love
2002-09-28 3:21 ` Andre Hedrick
2002-09-28 3:29 ` Robert Love
2002-09-28 10:06 ` Alan Cox
2002-09-28 17:06 ` Robert Love
2002-09-28 0:51 ` Robert Love
2002-09-28 2:16 ` Greg KH
2002-09-28 14:54 ` John Levon
2002-09-28 17:05 ` Andrew Morton
2002-09-28 17:24 ` John Levon
2002-09-28 18:27 ` Robert Love
2002-09-28 18:38 ` John Levon
2002-09-29 0:50 ` William Lee Irwin III [this message]
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