[Xen-devel] Re: [Patch] Fix SMP debugging assert failures in blktap

["Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>]

Applied, thank you.

a.

On 9/28/06, Stephen C. Tweedie <sct@xxxxxxxxxx> wrote:
> blktap is calling non-atomic kernel functions with irqs disabled, which
> hits various kernel debug traps if those are enabled.  The problem is
> req_increase(), which takes the pending_free_lock.
>
> This function is currently only ever called from module initialiation,
> where it is impossible for it to race against anything else.  Its
> companion, req_decrease(), is not called at all.
>
> Fix by removing the offending locking from req_increase() and, while
> we're at it, remove req_decrease() entirely.
>
> Signed-off-by: Stephen Tweedie <sct@xxxxxxxxxx>
>
> diff -r ed51caee4fe6 -r 94df5bd84195 
> linux-2.6-xen-sparse/drivers/xen/blktap/blktap.c
> --- a/linux-2.6-xen-sparse/drivers/xen/blktap/blktap.c  Thu Sep 28 15:38:25 
> 2006 +0100
> +++ b/linux-2.6-xen-sparse/drivers/xen/blktap/blktap.c  Thu Sep 28 17:09:09 
> 2006 +0100
> @@ -716,8 +716,6 @@ static int req_increase(void)
>         unsigned long flags;
>         int ret;
>
> -       spin_lock_irqsave(&pending_free_lock, flags);
> -
>         ret = -EINVAL;
>         if (mmap_alloc >= MAX_PENDING_REQS || mmap_lock)
>                 goto done;
> @@ -782,8 +780,7 @@ static int req_increase(void)
>
>         mmap_alloc++;
>         DPRINTK("# MMAPs increased to %d\n",mmap_alloc);
> - done:
> -       spin_unlock_irqrestore(&pending_free_lock, flags);
> +done:
>         return ret;
>  }
>
> @@ -811,36 +808,6 @@ static void mmap_req_del(int mmap)
>         mmap_lock = 0;
>         DPRINTK("# MMAPs decreased to %d\n",mmap_alloc);
>         mmap_alloc--;
> -}
> -
> -/*N.B. Currently unused - will be accessed via sysfs*/
> -static void req_decrease(void)
> -{
> -       pending_req_t *req;
> -       int i;
> -       unsigned long flags;
> -
> -       spin_lock_irqsave(&pending_free_lock, flags);
> -
> -       DPRINTK("Req decrease called.\n");
> -       if (mmap_lock || mmap_alloc == 1)
> -               goto done;
> -
> -       mmap_lock = 1;
> -       mmap_inuse = MAX_PENDING_REQS;
> -
> -        /*Go through reqs and remove any that aren't in use*/
> -       for (i = 0; i < MAX_PENDING_REQS ; i++) {
> -               req = &pending_reqs[mmap_alloc-1][i];
> -               if (req->inuse == 0) {
> -                       list_del(&req->free_list);
> -                       mmap_inuse--;
> -               }
> -       }
> -       if (mmap_inuse == 0) mmap_req_del(mmap_alloc-1);
> - done:
> -       spin_unlock_irqrestore(&pending_free_lock, flags);
> -       return;
>  }
>
>  static pending_req_t* alloc_req(void)
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