[PATCH] mtd: lpddr: Fix a potential double mutex_lock

[PATCH] mtd: lpddr: Fix a potential double mutex_lock

[Christophe JAILLET]

If 'chip->state == FL_SYNCING', we will 'goto retry' with the mutex
'&shared->lock' already taken.
In such a case, the 'mutex_lock' at line 927 can never succeed.

In order to avoid a deadlock, move the 'mutex_lock(&shared->lock)' at the
very end of the block.

This has been spotted with the following coccinelle script:

@find@
expression x, t;
@@

	mutex_lock(x);
	... when != mutex_unlock(x)
	mutex_lock(t);

@@
expression find.t;
expression find.x;
@@

*	mutex_lock(t);
	... when != mutex_unlock(t)
*	mutex_lock(x);


Fixes: 	c68264711ca6 ("[MTD] LPDDR Command set driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
Review carefuly, untested.
---
 drivers/mtd/lpddr/lpddr_cmds.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/lpddr/lpddr_cmds.c b/drivers/mtd/lpddr/lpddr_cmds.c
index 018c75faadb3..830dd0855ab3 100644
--- a/drivers/mtd/lpddr/lpddr_cmds.c
+++ b/drivers/mtd/lpddr/lpddr_cmds.c
@@ -237,7 +237,6 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
 				mutex_unlock(&contender->mutex);
 				return ret;
 			}
-			mutex_lock(&shared->lock);
 
 			/* We should not own chip if it is already in FL_SYNCING
 			 * state. Put contender and retry. */
@@ -247,6 +246,8 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
 				goto retry;
 			}
 			mutex_unlock(&contender->mutex);
+
+			mutex_lock(&shared->lock);
 		}
 
 		/* Check if we have suspended erase on this chip.
-- 
2.14.1

Re: [PATCH] mtd: lpddr: Fix a potential double mutex_lock

[Boris Brezillon]

Hi Christophe,

On Sun, 22 Oct 2017 10:28:31 +0200
Christophe JAILLET <christophe.jaillet@wanadoo.fr> wrote:

> If 'chip->state == FL_SYNCING', we will 'goto retry' with the mutex
> '&shared->lock' already taken.
> In such a case, the 'mutex_lock' at line 927 can never succeed.
> 
> In order to avoid a deadlock, move the 'mutex_lock(&shared->lock)' at the
> very end of the block.
> 
> This has been spotted with the following coccinelle script:
> 
> @find@
> expression x, t;
> @@
> 
> 	mutex_lock(x);
> 	... when != mutex_unlock(x)
> 	mutex_lock(t);
> 
> @@
> expression find.t;
> expression find.x;
> @@
> 
> *	mutex_lock(t);
> 	... when != mutex_unlock(t)
> *	mutex_lock(x);
> 
> 
> Fixes: 	c68264711ca6 ("[MTD] LPDDR Command set driver")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> Review carefuly, untested.
> ---
>  drivers/mtd/lpddr/lpddr_cmds.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/lpddr/lpddr_cmds.c b/drivers/mtd/lpddr/lpddr_cmds.c
> index 018c75faadb3..830dd0855ab3 100644
> --- a/drivers/mtd/lpddr/lpddr_cmds.c
> +++ b/drivers/mtd/lpddr/lpddr_cmds.c
> @@ -237,7 +237,6 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
>  				mutex_unlock(&contender->mutex);
>  				return ret;
>  			}
> -			mutex_lock(&shared->lock);
>  
>  			/* We should not own chip if it is already in FL_SYNCING
>  			 * state. Put contender and retry. */
> @@ -247,6 +246,8 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
>  				goto retry;
>  			}
>  			mutex_unlock(&contender->mutex);
> +
> +			mutex_lock(&shared->lock);

I had a quick look at this code and I must say the locking in this
driver is a real nightmare (apparently it was copied from CFI driver).

It's not entirely clear to me if the shared lock should be taken before
releasing the contender one (as done today) or if it can safely be
moved at the end of the block (as you suggest).
Unless we have someone who knows about the locking scheme of
cfi/lpddr and can confirm that moving the mutex_lock() at the end of
the block is safe, I'd recommend that we take a conservative approach
and add a mutex_unlock(&shared->lock) in the retry path and keep the
existing mutex_lock() in place.

>  		}
>  
>  		/* Check if we have suspended erase on this chip.

Re: [PATCH] mtd: lpddr: Fix a potential double mutex_lock

[Christophe JAILLET]

Le 31/10/2017 à 15:58, Boris Brezillon a écrit :
> Hi Christophe,
>
> On Sun, 22 Oct 2017 10:28:31 +0200
> Christophe JAILLET <christophe.jaillet@wanadoo.fr> wrote:
>
>> If 'chip->state == FL_SYNCING', we will 'goto retry' with the mutex
>> '&shared->lock' already taken.
>> In such a case, the 'mutex_lock' at line 927 can never succeed.
>>
>> In order to avoid a deadlock, move the 'mutex_lock(&shared->lock)' at the
>> very end of the block.
>>
>> This has been spotted with the following coccinelle script:
>>
>> @find@
>> expression x, t;
>> @@
>>
>> 	mutex_lock(x);
>> 	... when != mutex_unlock(x)
>> 	mutex_lock(t);
>>
>> @@
>> expression find.t;
>> expression find.x;
>> @@
>>
>> *	mutex_lock(t);
>> 	... when != mutex_unlock(t)
>> *	mutex_lock(x);
>>
>>
>> Fixes: 	c68264711ca6 ("[MTD] LPDDR Command set driver")
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
>> ---
>> Review carefuly, untested.
>> ---
>>   drivers/mtd/lpddr/lpddr_cmds.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/mtd/lpddr/lpddr_cmds.c b/drivers/mtd/lpddr/lpddr_cmds.c
>> index 018c75faadb3..830dd0855ab3 100644
>> --- a/drivers/mtd/lpddr/lpddr_cmds.c
>> +++ b/drivers/mtd/lpddr/lpddr_cmds.c
>> @@ -237,7 +237,6 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
>>   				mutex_unlock(&contender->mutex);
>>   				return ret;
>>   			}
>> -			mutex_lock(&shared->lock);
>>   
>>   			/* We should not own chip if it is already in FL_SYNCING
>>   			 * state. Put contender and retry. */
>> @@ -247,6 +246,8 @@ static int get_chip(struct map_info *map, struct flchip *chip, int mode)
>>   				goto retry;
>>   			}
>>   			mutex_unlock(&contender->mutex);
>> +
>> +			mutex_lock(&shared->lock);
> I had a quick look at this code and I must say the locking in this
> driver is a real nightmare (apparently it was copied from CFI driver).
>
> It's not entirely clear to me if the shared lock should be taken before
> releasing the contender one (as done today) or if it can safely be
> moved at the end of the block (as you suggest).
> Unless we have someone who knows about the locking scheme of
> cfi/lpddr and can confirm that moving the mutex_lock() at the end of
> the block is safe, I'd recommend that we take a conservative approach
> and add a mutex_unlock(&shared->lock) in the retry path and keep the
> existing mutex_lock() in place.
Agreed. I'll wait some time for any other feedback and will propose a 
more conservative patch in a couple of weeks if needed.
Thanks for the review.

CJ
>>   		}
>>   
>>   		/* Check if we have suspended erase on this chip.