Date: Wed, 21 Sep 2005 13:49:50 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: ru@freebsd.org Cc: cvs-src@freebsd.org, src-committers@freebsd.org, rwatson@freebsd.org, cvs-all@freebsd.org, jhb@freebsd.org Subject: Re: cvs commit: src/sys/dev/an if_an.c src/sys/dev/arl if_arl_isa.c src/sys/dev/awi if_awi_pccard.c src/sys/dev/cm if_cm_isa.c src/sys/dev/cnw if_cnw.c src/sys/dev/cp if_cp.c src/sys/dev/cs if_cs.c src/sys/dev/ed if_ed.c src/sys/dev/em if_em.c ... Message-ID: <20050921.134950.132929273.imp@bsdimp.com> In-Reply-To: <20050921193410.GC24516@ip.net.ua> References: <20050921191906.GA24516@ip.net.ua> <20050921.132322.116350250.imp@bsdimp.com> <20050921193410.GC24516@ip.net.ua>
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In message: <20050921193410.GC24516@ip.net.ua>
Ruslan Ermilov <ru@freebsd.org> writes:
: On Wed, Sep 21, 2005 at 01:23:22PM -0600, M. Warner Losh wrote:
: > In message: <20050921191906.GA24516@ip.net.ua>
: > Ruslan Ermilov <ru@FreeBSD.org> writes:
: > : On Wed, Sep 21, 2005 at 01:02:41PM -0600, M. Warner Losh wrote:
: > : > In message: <200509211455.59154.jhb@FreeBSD.org>
: > : > John Baldwin <jhb@FreeBSD.org> writes:
: > : > : > 5. Fix all drivers to set some flag in foo_detach() and foo_shutdown()
: > : > : > and refuse to work in foo_ioctl() if it's set. This should fix
: > : > : > panics when BPF listener is attached while interface goes away or
: > : > : > module is unloaded.
: > : >
: > : > : I'd rather 5) be simpler in
: > : > : that it only check in the flags case to not force the driver lock to be
: > : > : acquired for all the ioctls that the driver doesn't actually handle.
: > : > : Actually, I think I'd really prefer that we think about how to fix the BPF
: > : > : issue in BPF itself if possible. It may be that we don't need to set the
: > : > : flags (i.e. skip the actual ioctl) if the interface is in the process of
: > : > : detaching and we can make that change centrally without having to scatter
: > : > : gone flags in all the drivers.
: > : >
: > : > This would solve the race at hand. However, it wouldn't solve the
: > : > problems with driver shutdown racing with other things in the system
: > : > (like ifconfig during detach).
: > : >
: > : > I've knocked around the idea of creating a if_dead() function that one
: > : > would call before foo_stop(). if_dead would just return (possibly an
: > : > error) for all the entry points before the device is actually
: > : > detached. This would mean that we don't need to add flags to all the
: > : > drivers, but we do need to change all the detach routine. This would
: > : > avoid the LOCK operations that you are worried about...
: > : >
: > : I'm afraid you cannot safely change the ifnet method pointers, if
: > : you mean that foo_detach() should set ifp->if_start etc. to point
: > : to if_dead().
: >
: > Why not? What's the problem?
: >
: The time you change say ifp->if_start in foo_detach(), stack may call
: it and get a half-changed pointer. Unless we invent locking shared
: between stack and the drivers.
pointers can be changed atomically on all our platforms. The only
race is that there's a number of pointers that you need to change and
you can't change all of them at the same time. ifp->if_start will
either point to the old driver start routine, or it will point to the
new routine. It won't be 1/2 and 1/2.
Warner
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