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Make device_busy/unbusy work w/o Giant held

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The vast majority of the busy/unbusy users in the tree don't acquire
Giant before calling device_busy/unbusy. However, if multiple threads
are opening a file, say, that causes the device to busy/unbusy, then we
can race to the root marking things busy. Move to using a reference
count to keep track of how many times a device_t has been made busy. Use
that count to make the same decisions that we'd make with the old device
state.

Note: gpiopps.c uses D_TRACKCLOSE. Others do as well. However, there's a
known race with closes that will be corrected for all the drivers that
do this in a future commit.

Sponsored by:		Netflix
Reviewed by:		hselasky, jhb
Differential Revision:	https://reviews.freebsd.org/D26284
This commit is contained in:
Warner Losh 2021-11-30 15:03:26 -07:00
parent 25c49c426c
commit 1c7d15b030
5 changed files with 20 additions and 39 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
sys/dev/drm2/drm_fops.c
View File

@ -157,9 +157,7 @@ int drm_open(struct cdev *kdev, int flags, int fmt, DRM_STRUCTPROC *p)
return 0;
err_undo:
mtx_lock(&Giant); /* FIXME: Giant required? */
device_unbusy(dev->dev);
mtx_unlock(&Giant);
dev->open_count--;
sx_xunlock(&drm_global_mutex);
return -retcode;
@ -273,9 +271,7 @@ static int drm_open_helper(struct cdev *kdev, int flags, int fmt,
list_add(&priv->lhead, &dev->filelist);
DRM_UNLOCK(dev);
mtx_lock(&Giant); /* FIXME: Giant required? */
device_busy(dev->dev);
mtx_unlock(&Giant);
ret = devfs_set_cdevpriv(priv, drm_release);
if (ret != 0)
@ -453,9 +449,7 @@ void drm_release(void *data)
*/
atomic_inc(&dev->counts[_DRM_STAT_CLOSES]);
mtx_lock(&Giant);
device_unbusy(dev->dev);
mtx_unlock(&Giant);
if (!--dev->open_count) {
if (atomic_read(&dev->ioctl_count)) {
DRM_ERROR("Device busy: %d\n",

9
sys/dev/gpio/gpiopps.c
View File

@ -73,9 +73,7 @@ gpiopps_open(struct cdev *dev, int flags, int fmt, struct thread *td)
/* We can't be unloaded while open, so mark ourselves BUSY. */
mtx_lock(&sc->pps_mtx);
if (device_get_state(sc->dev) < DS_BUSY) {
device_busy(sc->dev);
}
device_busy(sc->dev);
mtx_unlock(&sc->pps_mtx);
return 0;
@ -86,10 +84,6 @@ gpiopps_close(struct cdev *dev, int flags, int fmt, struct thread *td)
{
struct pps_softc *sc = dev->si_drv1;
/*
* Un-busy on last close. We rely on the vfs counting stuff to only call
* this routine on last-close, so we don't need any open-count logic.
*/
mtx_lock(&sc->pps_mtx);
device_unbusy(sc->dev);
mtx_unlock(&sc->pps_mtx);
@ -113,6 +107,7 @@ gpiopps_ioctl(struct cdev *dev, u_long cmd, caddr_t data, int flags, struct thre
static struct cdevsw pps_cdevsw = {
.d_version = D_VERSION,
.d_flags = D_TRACKCLOSE,
.d_open = gpiopps_open,
.d_close = gpiopps_close,
.d_ioctl = gpiopps_ioctl,

2
sys/dev/pccard/pccard.c
View File

@ -342,7 +342,7 @@ pccard_detach_card(device_t dev)
if (pf->dev == NULL)
continue;
state = device_get_state(pf->dev);
if (state == DS_ATTACHED || state == DS_BUSY)
if (state == DS_ATTACHED)
device_detach(pf->dev);
if (pf->cfe != NULL)
pccard_function_disable(pf);

41
sys/kern/subr_bus.c
View File

@ -51,6 +51,7 @@ __FBSDID("$FreeBSD$");
#include <sys/queue.h>
#include <machine/bus.h>
#include <sys/random.h>
#include <sys/refcount.h>
#include <sys/rman.h>
#include <sys/sbuf.h>
#include <sys/selinfo.h>
@ -140,7 +141,7 @@ struct _device {
int unit; /**< current unit number */
char* nameunit; /**< name+unit e.g. foodev0 */
char* desc; /**< driver specific description */
int busy; /**< count of calls to device_busy() */
u_int busy; /**< count of calls to device_busy() */
device_state_t state; /**< current device state */
uint32_t devflags; /**< api level flags for device_get_flags() */
u_int flags; /**< internal device flags */
@ -2634,13 +2635,13 @@ device_disable(device_t dev)
void
device_busy(device_t dev)
{
if (dev->state < DS_ATTACHING)
panic("device_busy: called for unattached device");
if (dev->busy == 0 && dev->parent)
/*
* Mark the device as busy, recursively up the tree if this busy count
* goes 0->1.
*/
if (refcount_acquire(&dev->busy) == 0 && dev->parent != NULL)
device_busy(dev->parent);
dev->busy++;
if (dev->state == DS_ATTACHED)
dev->state = DS_BUSY;
}
/**
@ -2649,17 +2650,12 @@ device_busy(device_t dev)
void
device_unbusy(device_t dev)
{
if (dev->busy != 0 && dev->state != DS_BUSY &&
dev->state != DS_ATTACHING)
panic("device_unbusy: called for non-busy device %s",
device_get_nameunit(dev));
dev->busy--;
if (dev->busy == 0) {
if (dev->parent)
device_unbusy(dev->parent);
if (dev->state == DS_BUSY)
dev->state = DS_ATTACHED;
}
/*
* Mark the device as unbsy, recursively if this is the last busy count.
*/
if (refcount_release(&dev->busy) && dev->parent != NULL)
device_unbusy(dev->parent);
}
/**
@ -3004,10 +3000,7 @@ device_attach(device_t dev)
attachentropy = (uint16_t)(get_cyclecount() - attachtime);
random_harvest_direct(&attachentropy, sizeof(attachentropy), RANDOM_ATTACH);
device_sysctl_update(dev);
if (dev->busy)
dev->state = DS_BUSY;
else
dev->state = DS_ATTACHED;
dev->state = DS_ATTACHED;
dev->flags &= ~DF_DONENOMATCH;
EVENTHANDLER_DIRECT_INVOKE(device_attach, dev);
devadded(dev);
@ -3038,7 +3031,7 @@ device_detach(device_t dev)
GIANT_REQUIRED;
PDEBUG(("%s", DEVICENAME(dev)));
if (dev->state == DS_BUSY)
if (dev->busy > 0)
return (EBUSY);
if (dev->state == DS_ATTACHING) {
device_printf(dev, "device in attaching state! Deferring detach.\n");
@ -3090,7 +3083,7 @@ int
device_quiesce(device_t dev)
{
PDEBUG(("%s", DEVICENAME(dev)));
if (dev->state == DS_BUSY)
if (dev->busy > 0)
return (EBUSY);
if (dev->state != DS_ATTACHED)
return (0);

1
sys/sys/bus.h
View File

@ -58,7 +58,6 @@ typedef enum device_state {
DS_ALIVE = 20, /**< @brief probe succeeded */
DS_ATTACHING = 25, /**< @brief currently attaching */
DS_ATTACHED = 30, /**< @brief attach method called */
DS_BUSY = 40 /**< @brief device is open */
} device_state_t;
/**