Date: Fri, 22 May 2009 11:52:59 -0400 From: John Baldwin <jhb@freebsd.org> To: Scott Long <scottl@samsco.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, Attilio Rao <attilio@freebsd.org>, svn-src-head@freebsd.org, rwatson@freebsd.org, Kostik Belousov <kostikbel@gmail.com>, "M. Warner Losh" <imp@bsdimp.com> Subject: Re: svn commit: r192535 - head/sys/kern Message-ID: <200905221153.00147.jhb@freebsd.org> In-Reply-To: <4A16C6C2.4020506@samsco.org> References: <3bbf2fe10905210629p46c7a204v6863aaba77354462@mail.gmail.com> <200905221125.36813.jhb@freebsd.org> <4A16C6C2.4020506@samsco.org>
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On Friday 22 May 2009 11:37:38 am Scott Long wrote: > John Baldwin wrote: > > On Friday 22 May 2009 9:44:18 am Scott Long wrote: > >> John Baldwin wrote: > >>> On Thursday 21 May 2009 6:11:02 pm Attilio Rao wrote: > >>>> At this point I wonder what's the purpose of maintaining the sleeping > >>>> version for such functions? > >>> Actually, I still very much do not like using M_NOWAIT needlessly. I would > >>> much rather the solution for make_dev() be that the 1 or 2 places that need > >>> to do it with a mutex held instead queue a task to do the actual make_dev() > >>> in a taskqueue when no locks are held. This is basically what > >>> destroy_dev_sched() is doing. Perhaps a make_dev_sched() with a similar > >>> callback to be called on completion would be better. Having a device driver > >>> do all the work to setup the hardware only to fail to create a node in /dev > >>> so that userland can actually use it is pretty rediculous and useless. > >>> > >> It's a lot easier for me to handle a failure of make_dev in CAM than it > >> is to decouple the call to it. Please don't dictate policy. > > > > But what is there for CAM to handle? I would expect CAM to handle hardware > > events such as the devices arriving or leaving. A temporary memory shortage > > it not a hardware event. As a user, if I insert a USB stick when the system > > happens to be temporarily low on memory, is it more useful for the cdev to > > appear a few microseconds later from a deferred context once memory is > > available or for no device to ever appear at all? > > > > John, > > You yourself have been recently burned by not fully understanding the > complexity involved in CAM. By changing all of the periph drivers to > conform to an artificial policy limitation of the make_dev call, I face > a significant amount of time and effort to rewrite and test code paths > that are, unfortunately, highly complex and very fragile. Please just > make a simple concession. Are you referring to the sysctl thing? That was quite trivial to fix FWIW. I also do not see why make_dev_sched() with a callback won't work? We already have this exact policy limitation in many similar APIs such as if_attach(). Another thing to consider is that if you hold a lock while calling into other subsystems, that can result in your lock being held for a relatively "long" time which increases the chances for lock contention. -- John Baldwin
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