[patch 1/1] uml:fix ubd deadlock on SMP

[patch 1/1] uml:fix ubd deadlock on SMP

[blaisorblade_spam]

Trivial: don't lock the queue spinlock when called from the request function.
Since the faulty function must use spinlock in another case, double-case it.
And since we will never use both functions together, let no object code be
shared between them.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
---

 uml-linux-2.6.8.1-paolo/arch/um/drivers/ubd_kern.c |   36 ++++++++++++++++-----
 1 files changed, 28 insertions(+), 8 deletions(-)

diff -puN arch/um/drivers/ubd_kern.c~uml-fix-ubd-deadlock arch/um/drivers/ubd_kern.c
--- uml-linux-2.6.8.1/arch/um/drivers/ubd_kern.c~uml-fix-ubd-deadlock	2004-09-08 19:04:27.662926344 +0200
+++ uml-linux-2.6.8.1-paolo/arch/um/drivers/ubd_kern.c	2004-09-08 19:05:36.700431048 +0200
@@ -54,6 +54,7 @@
 #include "mem.h"
 #include "mem_kern.h"
 
+/*This is the queue lock. FIXME: make it per-UBD device.*/
 static spinlock_t ubd_io_lock = SPIN_LOCK_UNLOCKED;
 static spinlock_t ubd_lock = SPIN_LOCK_UNLOCKED;
 
@@ -396,14 +397,16 @@ int thread_fd = -1;
  */
 int intr_count = 0;
 
-static void ubd_finish(struct request *req, int error)
+static inline void __ubd_finish(struct request *req, int error, int lock)
 {
 	int nsect;
 
 	if(error){
- 		spin_lock(&ubd_io_lock);
+		if (lock)
+			spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
+		if (lock)
+			spin_unlock(&ubd_io_lock);
 		return;
 	}
 	nsect = req->current_nr_sectors;
@@ -412,11 +415,28 @@ static void ubd_finish(struct request *r
 	req->errors = 0;
 	req->nr_sectors -= nsect;
 	req->current_nr_sectors = 0;
-	spin_lock(&ubd_io_lock);
+	if (lock)
+		spin_lock(&ubd_io_lock);
 	end_request(req, 1);
-	spin_unlock(&ubd_io_lock);
+	if (lock)
+		spin_unlock(&ubd_io_lock);
+}
+
+/* We will use only one of them, not both, i.e. ubd_finish with the io_thread
+ * and ubd_finish_nolock without the separate io thread, so it's better to waste
+ * some space to gain performance. */
+
+static void ubd_finish(struct request *req, int error)
+{
+	__ubd_finish(req, error, 1);
+}
+
+static void ubd_finish_nolock(struct request *req, int error)
+{
+	__ubd_finish(req, error, 0);
 }
 
+/*Called with ubd_io_lock not held*/
 static void ubd_handler(void)
 {
 	struct io_thread_req req;
@@ -965,6 +985,7 @@ static int prepare_mmap_request(struct u
 	return(0);
 }
 
+/*Called with ubd_io_lock held*/
 static int prepare_request(struct request *req, struct io_thread_req *io_req)
 {
 	struct gendisk *disk = req->rq_disk;
@@ -977,9 +998,7 @@ static int prepare_request(struct reques
 	if((rq_data_dir(req) == WRITE) && !dev->openflags.w){
 		printk("Write attempted on readonly ubd device %s\n", 
 		       disk->disk_name);
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return(1);
 	}
 
@@ -1029,6 +1048,7 @@ static int prepare_request(struct reques
 	return(0);
 }
 
+/*Called with ubd_io_lock held*/
 static void do_ubd_request(request_queue_t *q)
 {
 	struct io_thread_req io_req;
@@ -1040,7 +1060,7 @@ static void do_ubd_request(request_queue
 			err = prepare_request(req, &io_req);
 			if(!err){
 				do_io(&io_req);
-				ubd_finish(req, io_req.error);
+				ubd_finish_nolock(req, io_req.error);
 			}
 		}
 	}
_

Re: [patch 1/1] uml:fix ubd deadlock on SMP

[Chris Wright]

* blaisorblade_spam@yahoo.it (blaisorblade_spam@yahoo.it) wrote:
> 
> Trivial: don't lock the queue spinlock when called from the request function.
> Since the faulty function must use spinlock in another case, double-case it.
> And since we will never use both functions together, let no object code be
> shared between them.

Why not add a helper which locks around the core function.  Then either
call helper or core function directly depending on locking needs?

Smth. along the lines of below.

===== arch/um/drivers/ubd_kern.c 1.36 vs edited =====
--- 1.36/arch/um/drivers/ubd_kern.c	2004-08-24 02:08:18 -07:00
+++ edited/arch/um/drivers/ubd_kern.c	2004-09-08 11:06:54 -07:00
@@ -396,14 +396,20 @@
  */
 int intr_count = 0;
 
-static void ubd_finish(struct request *req, int error)
+static inline void ubd_finish(struct request *req, int error)
+{
+ 	spin_lock(&ubd_io_lock);
+	__ubd_finish(req, error);
+	spin_unlock(&ubd_io_lock);
+}
+
+/* call ubd_finish if you need to serialize */
+static void __ubd_finish(struct request *req, int error)
 {
 	int nsect;
 
 	if(error){
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return;
 	}
 	nsect = req->current_nr_sectors;
@@ -412,9 +418,7 @@
 	req->errors = 0;
 	req->nr_sectors -= nsect;
 	req->current_nr_sectors = 0;
-	spin_lock(&ubd_io_lock);
 	end_request(req, 1);
-	spin_unlock(&ubd_io_lock);
 }
 
 static void ubd_handler(void)
-- 
Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net

Re: [patch 1/1] uml:fix ubd deadlock on SMP

[Jens Axboe]

On Wed, Sep 08 2004, blaisorblade_spam@yahoo.it wrote:
> diff -puN arch/um/drivers/ubd_kern.c~uml-fix-ubd-deadlock arch/um/drivers/ubd_kern.c
> --- uml-linux-2.6.8.1/arch/um/drivers/ubd_kern.c~uml-fix-ubd-deadlock	2004-09-08 19:04:27.662926344 +0200
> +++ uml-linux-2.6.8.1-paolo/arch/um/drivers/ubd_kern.c	2004-09-08 19:05:36.700431048 +0200
> @@ -54,6 +54,7 @@
>  #include "mem.h"
>  #include "mem_kern.h"
>  
> +/*This is the queue lock. FIXME: make it per-UBD device.*/
>  static spinlock_t ubd_io_lock = SPIN_LOCK_UNLOCKED;
>  static spinlock_t ubd_lock = SPIN_LOCK_UNLOCKED;

probably not worth it to make it per-device. doing so should be a simple
search-replace job, though.

> @@ -396,14 +397,16 @@ int thread_fd = -1;
>   */
>  int intr_count = 0;
>  
> -static void ubd_finish(struct request *req, int error)
> +static inline void __ubd_finish(struct request *req, int error, int lock)
>  {
>  	int nsect;
>  
>  	if(error){
> - 		spin_lock(&ubd_io_lock);
> +		if (lock)
> +			spin_lock(&ubd_io_lock);
>  		end_request(req, 0);
> - 		spin_unlock(&ubd_io_lock);
> +		if (lock)
> +			spin_unlock(&ubd_io_lock);

In general, doing it this way is throwned upon. Either split the
function, or just make the callers acquire the lock if they have to.

-- 
Jens Axboe

Re: [uml-devel] Re: [patch 1/1] uml:fix ubd deadlock on SMP

[BlaisorBlade]

On Wednesday 08 September 2004 20:12, Chris Wright wrote:
> * blaisorblade_spam@yahoo.it (blaisorblade_spam@yahoo.it) wrote:
> > Trivial: don't lock the queue spinlock when called from the request
> > function. Since the faulty function must use spinlock in another case,
> > double-case it. And since we will never use both functions together, let
> > no object code be shared between them.
>
> Why not add a helper which locks around the core function.  Then either
> call helper or core function directly depending on locking needs?
I'm happy with whatever is nicer.
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729

Re: [uml-devel] Re: [patch 1/1] uml:fix ubd deadlock on SMP

[Chris Wright]

* BlaisorBlade (blaisorblade_spam@yahoo.it) wrote:
> On Wednesday 08 September 2004 20:12, Chris Wright wrote:
> > * blaisorblade_spam@yahoo.it (blaisorblade_spam@yahoo.it) wrote:
> > > Trivial: don't lock the queue spinlock when called from the request
> > > function. Since the faulty function must use spinlock in another case,
> > > double-case it. And since we will never use both functions together, let
> > > no object code be shared between them.
> >
> > Why not add a helper which locks around the core function.  Then either
> > call helper or core function directly depending on locking needs?
> I'm happy with whatever is nicer.

The way I outlined is nicer as it avoids all that conditional locking.
I can do a full patch if you like.

thanks,
-chris
-- 
Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net

Re: [uml-devel] Re: [patch 1/1] uml:fix ubd deadlock on SMP

[BlaisorBlade]

On Thursday 09 September 2004 20:32, Chris Wright wrote:
> * BlaisorBlade (blaisorblade_spam@yahoo.it) wrote:
> > On Wednesday 08 September 2004 20:12, Chris Wright wrote:
> > > * blaisorblade_spam@yahoo.it (blaisorblade_spam@yahoo.it) wrote:
> > > > Trivial: don't lock the queue spinlock when called from the request
> > > > function. Since the faulty function must use spinlock in another
> > > > case, double-case it. And since we will never use both functions
> > > > together, let no object code be shared between them.
> > >
> > > Why not add a helper which locks around the core function.  Then either
> > > call helper or core function directly depending on locking needs?
> >
> > I'm happy with whatever is nicer.
>
> The way I outlined is nicer as it avoids all that conditional locking.
> I can do a full patch if you like.
Yes, thanks a lot for your help.
Bye
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729

Re: [uml-devel] Re: [patch 1/1] uml:fix ubd deadlock on SMP

[Chris Wright]

* BlaisorBlade (blaisorblade_spam@yahoo.it) wrote:
> Yes, thanks a lot for your help.

Rename ubd_finish() to __ubd_finsh() and remove ubd_io_lock from it.
Add wrapper, ubd_finish(), which grabs lock before calling __ubd_finish().
Update do_ubd_request to use the lock free __ubd_finish() to avoid
deadlock.  Also, apparently prepare_request is called with ubd_io_lock
held, so remove locks there.

Signed-off-by: Chris Wright <chrisw@osdl.org>

===== arch/um/drivers/ubd_kern.c 1.38 vs edited =====
--- 1.38/arch/um/drivers/ubd_kern.c	2004-09-07 23:33:13 -07:00
+++ edited/arch/um/drivers/ubd_kern.c	2004-09-09 12:18:01 -07:00
@@ -396,14 +396,20 @@
  */
 int intr_count = 0;
 
-static void ubd_finish(struct request *req, int error)
+static inline void ubd_finish(struct request *req, int error)
+{
+ 	spin_lock(&ubd_io_lock);
+	__ubd_finish(req, error);
+	spin_unlock(&ubd_io_lock);
+}
+
+/* call ubd_finish if you need to serialize */
+static void __ubd_finish(struct request *req, int error)
 {
 	int nsect;
 
 	if(error){
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return;
 	}
 	nsect = req->current_nr_sectors;
@@ -412,11 +418,10 @@
 	req->errors = 0;
 	req->nr_sectors -= nsect;
 	req->current_nr_sectors = 0;
-	spin_lock(&ubd_io_lock);
 	end_request(req, 1);
-	spin_unlock(&ubd_io_lock);
 }
 
+/* Called without ubd_io_lock held */
 static void ubd_handler(void)
 {
 	struct io_thread_req req;
@@ -965,6 +970,7 @@
 	return(0);
 }
 
+/* Called with ubd_io_lock held */
 static int prepare_request(struct request *req, struct io_thread_req *io_req)
 {
 	struct gendisk *disk = req->rq_disk;
@@ -977,9 +983,7 @@
 	if((rq_data_dir(req) == WRITE) && !dev->openflags.w){
 		printk("Write attempted on readonly ubd device %s\n", 
 		       disk->disk_name);
- 		spin_lock(&ubd_io_lock);
 		end_request(req, 0);
- 		spin_unlock(&ubd_io_lock);
 		return(1);
 	}
 
@@ -1029,6 +1033,7 @@
 	return(0);
 }
 
+/* Called with ubd_io_lock held */
 static void do_ubd_request(request_queue_t *q)
 {
 	struct io_thread_req io_req;
@@ -1040,7 +1045,7 @@
 			err = prepare_request(req, &io_req);
 			if(!err){
 				do_io(&io_req);
-				ubd_finish(req, io_req.error);
+				__ubd_finish(req, io_req.error);
 			}
 		}
 	}

Re: [uml-devel] Re: [patch 1/1] uml:fix ubd deadlock on SMP

[BlaisorBlade]

On Thursday 09 September 2004 21:29, Chris Wright wrote:
> * BlaisorBlade (blaisorblade_spam@yahoo.it) wrote:
> > Yes, thanks a lot for your help.
>
> Rename ubd_finish() to __ubd_finsh() and remove ubd_io_lock from it.
> Add wrapper, ubd_finish(), which grabs lock before calling __ubd_finish().
> Update do_ubd_request to use the lock free __ubd_finish() to avoid
> deadlock.  Also, apparently prepare_request is called with ubd_io_lock
> held, so remove locks there.

> Signed-off-by: Chris Wright <chrisw@osdl.org>
Ok, this is good. And it's the only one which has been discussed upon, Andrew, 
so you can merge the rest.
> ===== arch/um/drivers/ubd_kern.c 1.38 vs edited =====
> --- 1.38/arch/um/drivers/ubd_kern.c	2004-09-07 23:33:13 -07:00
> +++ edited/arch/um/drivers/ubd_kern.c	2004-09-09 12:18:01 -07:00
> @@ -396,14 +396,20 @@
>   */
>  int intr_count = 0;
>
> -static void ubd_finish(struct request *req, int error)
> +static inline void ubd_finish(struct request *req, int error)
> +{
> + 	spin_lock(&ubd_io_lock);
> +	__ubd_finish(req, error);
> +	spin_unlock(&ubd_io_lock);
> +}
> +
> +/* call ubd_finish if you need to serialize */
> +static void __ubd_finish(struct request *req, int error)
>  {
>  	int nsect;
>
>  	if(error){
> - 		spin_lock(&ubd_io_lock);
>  		end_request(req, 0);
> - 		spin_unlock(&ubd_io_lock);
>  		return;
>  	}
>  	nsect = req->current_nr_sectors;
> @@ -412,11 +418,10 @@
>  	req->errors = 0;
>  	req->nr_sectors -= nsect;
>  	req->current_nr_sectors = 0;
> -	spin_lock(&ubd_io_lock);
>  	end_request(req, 1);
> -	spin_unlock(&ubd_io_lock);
>  }
>
> +/* Called without ubd_io_lock held */
>  static void ubd_handler(void)
>  {
>  	struct io_thread_req req;
> @@ -965,6 +970,7 @@
>  	return(0);
>  }
>
> +/* Called with ubd_io_lock held */
>  static int prepare_request(struct request *req, struct io_thread_req
> *io_req) {
>  	struct gendisk *disk = req->rq_disk;
> @@ -977,9 +983,7 @@
>  	if((rq_data_dir(req) == WRITE) && !dev->openflags.w){
>  		printk("Write attempted on readonly ubd device %s\n",
>  		       disk->disk_name);
> - 		spin_lock(&ubd_io_lock);
>  		end_request(req, 0);
> - 		spin_unlock(&ubd_io_lock);
>  		return(1);
>  	}
>
> @@ -1029,6 +1033,7 @@
>  	return(0);
>  }
>
> +/* Called with ubd_io_lock held */
>  static void do_ubd_request(request_queue_t *q)
>  {
>  	struct io_thread_req io_req;
> @@ -1040,7 +1045,7 @@
>  			err = prepare_request(req, &io_req);
>  			if(!err){
>  				do_io(&io_req);
> -				ubd_finish(req, io_req.error);
> +				__ubd_finish(req, io_req.error);
>  			}
>  		}
>  	}
>
>
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-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729