[PATCH] drm: Remove count_lock for calling lastclose() after 58474713 (v2)

[Chris Wilson]

When removing of the BKL the locking around lastclose() was rearranged
and resulted in the holding of the open_count spinlock over the call
into drm_lastclose(). The drivers were not ready for this path to be
atomic - it may indeed involve long waits to release old objects and
cleanup the GPU - and so we ended up scheduling whilst atomic.

[   54.625598] BUG: scheduling while atomic: X/3546/0x00000002
[   54.625600] Modules linked in: sco bridge stp llc input_polldev rfcomm bnep l2cap crc16 sch_sfq ipv6 md_mod acpi_cpufreq mperf cryptd aes_x86_64 aes_generic xts gf128mul dm_crypt dm_mod btusb bluetooth usbhid hid zaurus cdc_ether usbnet mii cdc_wdm cdc_acm uvcvideo videodev v4l1_compat v4l2_compat_ioctl32 snd_hda_codec_conexant arc4 pcmcia ecb snd_hda_intel joydev sdhci_pci sdhci snd_hda_codec tpm_tis firewire_ohci mmc_core e1000e uhci_hcd thinkpad_acpi nvram yenta_socket pcmcia_rsrc pcmcia_core tpm wmi sr_mod firewire_core iwlagn ehci_hcd snd_hwdep snd_pcm usbcore tpm_bios thermal led_class snd_timer iwlcore snd soundcore ac snd_page_alloc pcspkr psmouse serio_raw battery sg mac80211 evdev cfg80211 i2c_i801 iTCO_wdt iTCO_vendor_support cdrom processor crc_itu_t rfkill xfs exportfs sd_mod crc_t10dif ahci libahci libata scsi_mod [last unloaded: scsi_wait_scan]
[   54.625663] Pid: 3546, comm: X Not tainted 2.6.35-04771-g1787985 #301
[   54.625665] Call Trace:
[   54.625671]  [<ffffffff8102d599>] __schedule_bug+0x57/0x5c
[   54.625675]  [<ffffffff81384141>] schedule+0xe5/0x832
[   54.625679]  [<ffffffff81163e77>] ? put_dec+0x20/0x3c
[   54.625682]  [<ffffffff81384dd4>] schedule_timeout+0x275/0x29f
[   54.625686]  [<ffffffff810455e1>] ? process_timeout+0x0/0xb
[   54.625688]  [<ffffffff81384e17>] schedule_timeout_uninterruptible+0x19/0x1b
[   54.625691]  [<ffffffff81045893>] msleep+0x16/0x1d
[   54.625695]  [<ffffffff812a2e53>] i9xx_crtc_dpms+0x273/0x2ae
[   54.625698]  [<ffffffff812a18be>] intel_crtc_dpms+0x28/0xe7
[   54.625702]  [<ffffffff811ec0fa>] drm_helper_disable_unused_functions+0xf0/0x118
[   54.625705]  [<ffffffff811ecde3>] drm_crtc_helper_set_config+0x644/0x7c8
[   54.625708]  [<ffffffff811f12dd>] ? drm_copy_field+0x40/0x50
[   54.625711]  [<ffffffff811ebca2>] drm_fb_helper_force_kernel_mode+0x3e/0x85
[   54.625713]  [<ffffffff811ebcf2>] drm_fb_helper_restore+0x9/0x24
[   54.625717]  [<ffffffff81290a41>] i915_driver_lastclose+0x2b/0x5c
[   54.625720]  [<ffffffff811f14a7>] drm_lastclose+0x44/0x2ad
[   54.625722]  [<ffffffff811f1ed2>] drm_release+0x5c6/0x609
[   54.625726]  [<ffffffff810d1275>] fput+0x109/0x1c7
[   54.625728]  [<ffffffff810ce5e4>] filp_close+0x61/0x6b
[   54.625731]  [<ffffffff810ce680>] sys_close+0x92/0xd4
[   54.625734]  [<ffffffff81002a2b>] system_call_fastpath+0x16/0x1b

v2: The spinlock is actually superfluous as access to open_count is
entirely serialised by drm_global_mutex and so can be dropped. The
count_lock spinlock instead appears to be used to protect access to
dev->buf_alloc and dev->buf_use.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/drm_fops.c |   16 +++-------------
 1 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
index 2ca8df8..3a652a6 100644
--- a/drivers/gpu/drm/drm_fops.c
+++ b/drivers/gpu/drm/drm_fops.c
@@ -135,15 +135,9 @@ int drm_open(struct inode *inode, struct file *filp)
 	retcode = drm_open_helper(inode, filp, dev);
 	if (!retcode) {
 		atomic_inc(&dev->counts[_DRM_STAT_OPENS]);
-		spin_lock(&dev->count_lock);
-		if (!dev->open_count++) {
-			spin_unlock(&dev->count_lock);
+		if (!dev->open_count++)
 			retcode = drm_setup(dev);
-			goto out;
-		}
-		spin_unlock(&dev->count_lock);
 	}
-out:
 	if (!retcode) {
 		mutex_lock(&dev->struct_mutex);
 		if (minor->type == DRM_MINOR_LEGACY) {
@@ -570,18 +564,14 @@ int drm_release(struct inode *inode, struct file *filp)
 	 */
 
 	atomic_inc(&dev->counts[_DRM_STAT_CLOSES]);
-	spin_lock(&dev->count_lock);
 	if (!--dev->open_count) {
 		if (atomic_read(&dev->ioctl_count)) {
 			DRM_ERROR("Device busy: %d\n",
 				  atomic_read(&dev->ioctl_count));
 			retcode = -EBUSY;
-			goto out;
-		}
-		retcode = drm_lastclose(dev);
+		} else
+			retcode = drm_lastclose(dev);
 	}
-out:
-	spin_unlock(&dev->count_lock);
 	mutex_unlock(&drm_global_mutex);
 
 	return retcode;
-- 
1.7.1