Date: Sun, 24 Nov 2019 15:10:10 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Warner Losh <imp@FreeBSD.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r355037 - head/sys/dev/pci Message-ID: <20191124131010.GB2707@kib.kiev.ua> In-Reply-To: <201911232343.xANNhqkQ097797@repo.freebsd.org> References: <201911232343.xANNhqkQ097797@repo.freebsd.org>
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On Sat, Nov 23, 2019 at 11:43:52PM +0000, Warner Losh wrote: > Author: imp > Date: Sat Nov 23 23:43:52 2019 > New Revision: 355037 > URL: https://svnweb.freebsd.org/changeset/base/355037 > > Log: > Push Giant down one layer > > The /dev/pci device doesn't need GIANT, per se. However, one routine > that it calls, pci_find_dbsf implicitly does. It walks a list that can > change when PCI scans a new bus. With hotplug, this means we could > have a race with that scanning. To prevent that, take out Giant around > scanning the list. > > However, given that we have places in the tree that drop giant, if > held when we call into them, the whole use of Giant to protect newbus > may be less effective that we desire, so add a comment about why we're > talking it out, and we'll address the issue when we lock newbus with > something other than Giant. > > Modified: > head/sys/dev/pci/pci.c > head/sys/dev/pci/pci_user.c > > Modified: head/sys/dev/pci/pci.c > ============================================================================== > --- head/sys/dev/pci/pci.c Sat Nov 23 23:41:21 2019 (r355036) > +++ head/sys/dev/pci/pci.c Sat Nov 23 23:43:52 2019 (r355037) > @@ -445,18 +445,21 @@ pci_find_bsf(uint8_t bus, uint8_t slot, uint8_t func) > device_t > pci_find_dbsf(uint32_t domain, uint8_t bus, uint8_t slot, uint8_t func) > { > - struct pci_devinfo *dinfo; > + struct pci_devinfo *dinfo = NULL; > > + /* Giant because newbus is Giant locked revisit with newbus locking */ > + mtx_lock(&Giant); > STAILQ_FOREACH(dinfo, &pci_devq, pci_links) { > if ((dinfo->cfg.domain == domain) && > (dinfo->cfg.bus == bus) && > (dinfo->cfg.slot == slot) && > (dinfo->cfg.func == func)) { > - return (dinfo->cfg.dev); > + break; > } > } > + mtx_unlock(&Giant); > > - return (NULL); > + return (dinfo != NULL ? dinfo->cfg.dev : NULL); I do not think this change is correct. If the parallel hotplug, or rather, hot-unplug event occurs, then dinfo potentially becomes invalid right after the Giant unlock, which makes both this function and its callers to access freed memory. Having caller to lock a newbus lock around both the call and consumption of the returned data is required. > } > > /* Find a device_t by vendor/device ID */ > > Modified: head/sys/dev/pci/pci_user.c > ============================================================================== > --- head/sys/dev/pci/pci_user.c Sat Nov 23 23:41:21 2019 (r355036) > +++ head/sys/dev/pci/pci_user.c Sat Nov 23 23:43:52 2019 (r355037) > @@ -119,7 +119,7 @@ static d_ioctl_t pci_ioctl; > > struct cdevsw pcicdev = { > .d_version = D_VERSION, > - .d_flags = D_NEEDGIANT, > + .d_flags = 0, > .d_open = pci_open, > .d_close = pci_close, > .d_ioctl = pci_ioctl,
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