On Sat, 23 Feb 2008, Alan Stern wrote:
> It happened in a workqueue. There could be lots of similar cases: Some
> interrupt-driven event causes a hotplug action. Since the action can't
> be carried out in interrupt context, the driver has no choice but to
> defer it to a workqueue or kernel thread. Blocking that workqueue or
> kernel thread won't cause a problem _unless_ the driver's
> suspend/resume methods try to synchronize with it. That's the
> difficult case.
In fact this turns out to be exactly the problem with the MMC
subsystem. It uses a dedicated workqueue (kmmcd) to handle state
changes, and mmc_suspend_host() does a flush_workqueue(). If the
workqueue contains an entry to register or unregister a device, this is
guaranteed to deadlock -- and that's exactly what happens when Zdenek
inserts or removes a card during the disk-drive spindown time of a
system sleep.
It looks like the best solution is for mmc_suspend_host() not to flush
the workqueue; instead the workqueue should prevent itself from
running during a suspend. Right now the easiest way to accomplish this
is for the workqueue routines (mmc_detect() and siblings) to acquire
the mmc_host's device semaphore. If in the future the MMC subsystem
adds dynamic PM support then a more complicated arrangement will be
needed.
So the patch below, in combination with the previous patch, might just
fix the whole problem.
Alan Stern
Index: usb-2.6/drivers/mmc/core/core.c
===================================================================
--- usb-2.6.orig/drivers/mmc/core/core.c
+++ usb-2.6/drivers/mmc/core/core.c
@@ -731,8 +731,6 @@ void mmc_stop_host(struct mmc_host *host
*/
int mmc_suspend_host(struct mmc_host *host, pm_message_t state)
{
- mmc_flush_scheduled_work();
-
mmc_bus_get(host);
if (host->bus_ops && !host->bus_dead) {
if (host->bus_ops->suspend)
Index: usb-2.6/drivers/mmc/core/mmc.c
===================================================================
--- usb-2.6.orig/drivers/mmc/core/mmc.c
+++ usb-2.6/drivers/mmc/core/mmc.c
@@ -441,6 +441,7 @@ static void mmc_detect(struct mmc_host *
BUG_ON(!host);
BUG_ON(!host->card);
+ down(&mmc_classdev(host)->sem);
mmc_claim_host(host);
/*
@@ -449,6 +450,7 @@ static void mmc_detect(struct mmc_host *
err = mmc_send_status(host->card, NULL);
mmc_release_host(host);
+ up(&mmc_classdev(host)->sem);
if (err) {
mmc_remove(host);
Index: usb-2.6/drivers/mmc/core/sd.c
===================================================================
--- usb-2.6.orig/drivers/mmc/core/sd.c
+++ usb-2.6/drivers/mmc/core/sd.c
@@ -500,6 +500,7 @@ static void mmc_sd_detect(struct mmc_hos
BUG_ON(!host);
BUG_ON(!host->card);
+ down(&mmc_classdev(host)->sem);
mmc_claim_host(host);
/*
@@ -508,6 +509,7 @@ static void mmc_sd_detect(struct mmc_hos
err = mmc_send_status(host->card, NULL);
mmc_release_host(host);
+ up(&mmc_classdev(host)->sem);
if (err) {
mmc_sd_remove(host);
Index: usb-2.6/drivers/mmc/core/sdio.c
===================================================================
--- usb-2.6.orig/drivers/mmc/core/sdio.c
+++ usb-2.6/drivers/mmc/core/sdio.c
@@ -195,6 +195,7 @@ static void mmc_sdio_detect(struct mmc_h
BUG_ON(!host);
BUG_ON(!host->card);
+ down(&mmc_classdev(host)->sem);
mmc_claim_host(host);
/*
@@ -203,6 +204,7 @@ static void mmc_sdio_detect(struct mmc_h
err = mmc_select_card(host->card);
mmc_release_host(host);
+ up(&mmc_classdev(host)->sem);
if (err) {
mmc_sdio_remove(host);
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