Messages in this thread Patch in this message | | | Date | Fri, 25 Jan 2008 16:23:56 +0000 | From | Ian Abbott <> | Subject | [PATCH(v3) 2.6.24] Fix fakephp deadlock |
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From: Ian Abbott <abbotti@mev.co.uk>
This is the third version of my patch to fix the problem of a process deadlocking itself when it uses the fakephp driver to fake the removal of a PCI device.
This section describes the changes since the first version of the patch. Skip to the next section (after the "---") for the original rationale and (slightly modified) patch description, and for the patch itself.
Greg KH hinted at a race condition in the first version of the patch. In the first version, the enable_slot() function checked that the slot was still valid and then flushed the work queue to perform any pending removals of slots from sysfs before it finally rescanned the PCI buses. Another task could have come in and marked the slot as disabled after the validity check but before flushing the work queue, and this would cause the task to deadlock while flushing the work queue (for similar reasons to the problem I was initially trying to fix). (The sysfs attribute file's buffer semaphore wouldn't necessarily protect against that, because writing "0" to the "power" file can trigger a cascade of other slot removals that don't have that protection.)
The second version of the patch rewrote the enable_slot() function so it no longer flushes the work queue. To maintain the proper sequencing, it allocated and queued a work queue item to perform the PCI rescan (the work queue is single-threaded). I also removed the slot validity check as we don't really care which slot was used to trigger a rescan.
This third version of the patch uses a single statically allocated work queue item to perform the PCI rescan instead of allocating one dynamically in the enable_slot() function. When enable_slot() is called, it first cancels any queued PCI rescan work item and then requeues it. This means a previously scheduled PCI rescan may be skipped, but that shouldn't matter. The important thing is that the new PCI rescan is performed after any previously scheduled removals of PCI slots from sysfs.
Greg KH also seemed to have a problem with my use of unsigned long for the 'removed' member of 'struct dummy_slot'. I need that for the test_and_set_bit() and a narrower type wouldn't make 'struct dummy_slot' any smaller (and there are other ways to save space in 'struct dummy_slot' if it is a problem, but they make the code trickier).
On with the rationale and patch description....
--- From: Ian Abbott <abbotti@mev.co.uk>
If the fakephp driver is used to emulate removal of a PCI device by writing text string "0" to the "power" sysfs attribute file, this causes its parent directory and its contents (including the "power" file) to be deleted before the write operation returns. Unfortunately, it ends up in a deadlock waiting for itself to complete.
The deadlock is as follows: sysfs_write_file calls flush_write_buffer which calls sysfs_get_active_two before calling power_write_file in pci_hotplug_core.c via the sysfs store operation. The power_write_file function calls disable_slot in fakephp.c via the slot operation. The disable_slot function calls remove_slot which calls pci_hp_deregister (back in pci_hotplug_core.c) which calls fs_remove_slot which calls sysfs_remove_file to remove the "power" file. The sysfs_remove_file function calls sysfs_hash_and_remove which calls sysfs_addrm_finish which calls sysfs_deactivate. The sysfs_deactivate function sees that something has an active reference on the sysfs_dirent (from the previous call to sysfs_get_active_two back up the call stack somewhere) so waits for the active reference to go away, which is of course impossible.
The problem has been present since 2.6.21.
This patch breaks the deadlock by queuing work queue items on a single- threaded work queue to remove a slot from sysfs, and to rescan the PCI buses. There is also some protection against disabling a slot that is already being removed.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk> ---
--- linux-2.6.24/drivers/pci/hotplug/fakephp.c.orig 2008-01-24 22:58:37.000000000 +0000 +++ linux-2.6.24/drivers/pci/hotplug/fakephp.c 2008-01-25 15:58:21.000000000 +0000 @@ -39,6 +39,7 @@ #include <linux/init.h> #include <linux/string.h> #include <linux/slab.h> +#include <linux/workqueue.h> #include "../pci.h"
#if !defined(MODULE) @@ -63,10 +64,16 @@ struct dummy_slot { struct list_head node; struct hotplug_slot *slot; struct pci_dev *dev; + struct work_struct remove_work; + unsigned long removed; };
static int debug; static LIST_HEAD(slot_list); +static struct workqueue_struct *dummyphp_wq; + +static void pci_rescan_worker(struct work_struct *work); +static DECLARE_WORK(pci_rescan_work, pci_rescan_worker);
static int enable_slot (struct hotplug_slot *slot); static int disable_slot (struct hotplug_slot *slot); @@ -109,7 +116,7 @@ static int add_slot(struct pci_dev *dev) slot->name = &dev->dev.bus_id[0]; dbg("slot->name = %s\n", slot->name);
- dslot = kmalloc(sizeof(struct dummy_slot), GFP_KERNEL); + dslot = kzalloc(sizeof(struct dummy_slot), GFP_KERNEL); if (!dslot) goto error_info;
@@ -164,6 +171,14 @@ static void remove_slot(struct dummy_slo err("Problem unregistering a slot %s\n", dslot->slot->name); }
+/* called from the single-threaded workqueue handler to remove a slot */ +static void remove_slot_worker(struct work_struct *work) +{ + struct dummy_slot *dslot = + container_of(work, struct dummy_slot, remove_work); + remove_slot(dslot); +} + /** * pci_rescan_slot - Rescan slot * @temp: Device template. Should be set: bus and devfn. @@ -267,11 +282,17 @@ static inline void pci_rescan(void) { pci_rescan_buses(&pci_root_buses); }
+/* called from the single-threaded workqueue handler to rescan all pci buses */ +static void pci_rescan_worker(struct work_struct *work) +{ + pci_rescan(); +}
static int enable_slot(struct hotplug_slot *hotplug_slot) { /* mis-use enable_slot for rescanning of the pci bus */ - pci_rescan(); + cancel_work_sync(&pci_rescan_work); + queue_work(dummyphp_wq, &pci_rescan_work); return -ENODEV; }
@@ -306,6 +327,10 @@ static int disable_slot(struct hotplug_s err("Can't remove PCI devices with other PCI devices behind it yet.\n"); return -ENODEV; } + if (test_and_set_bit(0, &dslot->removed)) { + dbg("Slot already scheduled for removal\n"); + return -ENODEV; + } /* search for subfunctions and disable them first */ if (!(dslot->dev->devfn & 7)) { for (func = 1; func < 8; func++) { @@ -328,8 +353,9 @@ static int disable_slot(struct hotplug_s /* remove the device from the pci core */ pci_remove_bus_device(dslot->dev);
- /* blow away this sysfs entry and other parts. */ - remove_slot(dslot); + /* queue work item to blow away this sysfs entry and other parts. */ + INIT_WORK(&dslot->remove_work, remove_slot_worker); + queue_work(dummyphp_wq, &dslot->remove_work);
return 0; } @@ -340,6 +366,7 @@ static void cleanup_slots (void) struct list_head *next; struct dummy_slot *dslot;
+ destroy_workqueue(dummyphp_wq); list_for_each_safe (tmp, next, &slot_list) { dslot = list_entry (tmp, struct dummy_slot, node); remove_slot(dslot); @@ -351,6 +378,10 @@ static int __init dummyphp_init(void) { info(DRIVER_DESC "\n");
+ dummyphp_wq = create_singlethread_workqueue(MY_NAME); + if (!dummyphp_wq) + return -ENOMEM; + return pci_scan_buses(); }
-- -=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@mev.co.uk> )=- -=( Tel: +44 (0)161 477 1898 FAX: +44 (0)161 718 3587 )=-
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