Re: [PATCH] shm_destroy lock hang

[Marcelo Tosatti]

Hi,

Just please avoid doing that locking nastyness:

function() {
unlock();
}

lock();
if (something)
	function();
else
	unlock();

For this case its OK, but please avoid that.

On Thu, 27 Jun 2002, Hugh Dickins wrote:

> Martin reported "Bug with shared memory" to LKML 14 May: hang due to
> schedule in truncate_list_pages called from .... shm_destroy holding
> the shm_lock spinlock.  shm_destroy needs that lock for shm_rmid, but
> it can be safely unlocked once link from id to shp has been removed.
> Patch against 2.4.19-rc1 or -pre10-ac2, applies at offset to 2.5.24.
>
> --- 2.4.19-rc1/ipc/shm.c	Mon Jun 24 19:14:58 2002
> +++ linux/ipc/shm.c	Thu Jun 27 19:34:51 2002
> @@ -117,12 +117,14 @@
>   *
>   * @shp: struct to free
>   *
> - * It has to be called with shp and shm_ids.sem locked
> + * It has to be called with shp and shm_ids.sem locked,
> + * but returns with shp unlocked and freed.
>   */
>  static void shm_destroy (struct shmid_kernel *shp)
>  {
>  	shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;
>  	shm_rmid (shp->id);
> +	shm_unlock(shp->id);
>  	shmem_lock(shp->shm_file, 0);
>  	fput (shp->shm_file);
>  	kfree (shp);
> @@ -150,8 +152,8 @@
>  	if(shp->shm_nattch == 0 &&
>  	   shp->shm_flags & SHM_DEST)
>  		shm_destroy (shp);
> -
> -	shm_unlock(id);
> +	else
> +		shm_unlock(id);
>  	up (&shm_ids.sem);
>  }
>
> @@ -511,11 +513,9 @@
>  			shp->shm_flags |= SHM_DEST;
>  			/* Do not find it any more */
>  			shp->shm_perm.key = IPC_PRIVATE;
> +			shm_unlock(shmid);
>  		} else
>  			shm_destroy (shp);
> -
> -		/* Unlock */
> -		shm_unlock(shmid);
>  		up(&shm_ids.sem);
>  		return err;
>  	}
> @@ -653,7 +653,8 @@
>  	if(shp->shm_nattch == 0 &&
>  	   shp->shm_flags & SHM_DEST)
>  		shm_destroy (shp);
> -	shm_unlock(shmid);
> +	else
> +		shm_unlock(shmid);
>  	up (&shm_ids.sem);
>
>  	*raddr = (unsigned long) user_addr;
>