Re: [PATCH] Fix file lookup without ref

[Suzanne Wood <suzannew@cs.pdx.edu>]

Do you mind explaining what you mean by "don't hold a reference"
in the places you replace rcu_read_lock() with spin_lock() in
settings with nested fcheck_files() or files_fdtable() which 
in turn call rcu_dereference()?  How, for example, are the 
occurences in proc_readfd() and tid_fd_revalidate() in 
fs/proc/base.c different?  tid_fid_revalidate() doesn't make
a local assignment and has the FASTCALL put_files_struct, but
is there reasoning that proc_readfd() isn't similar to steal_locks()
in fs/locks.c?

Thanks.
Suzanne 

 > From: Dipankar Sarma 2006-04-12 18:43:06
 > 
 > This patch fixes a problem with some places in the kernel where
 > we look up file structure from the fd table but don't hold
 > a reference to the file. Those places cannot be lock-free.
 > These places aren't in fast path, so it is not a problem.
 > I have tested this patch on powerpc and x86_64 using basic
 > tests and ltp. We should aim to merge this for 2.6.17.
 > 
 > Thanks
 > Dipankar
 > 
 > 
 > There are places in the kernel where we look up files in fd tables 
 > and access the file structure without holding refereces to the file. 
 > So, we need special care to avoid the race between
 > looking up files in the fd table and tearing down of the file
 > in another CPU. Otherwise, one might see a NULL f_dentry or
 > such torn down version of the file. This patch fixes those
 > special places where such a race may happen.
 > 
 > Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
 > ---
 > 
 > 
 >  drivers/char/tty_io.c |    8 ++++++--
 >  fs/locks.c            |    9 +++++++--
 >  fs/proc/base.c        |   21 +++++++++++++++------
 >  3 files changed, 28 insertions(+), 10 deletions(-)
 > 
 > diff -puN drivers/char/tty_io.c~fix-proc-fd-ops drivers/char/tty_io.c
 > --- linux-2.6.16-rcu/drivers/char/tty_io.c~fix-proc-fd-ops	2006-04-12 21:06:24.000000000 +0530
 > +++ linux-2.6.16-rcu-dipankar/drivers/char/tty_io.c	2006-04-12 21:06:24.000000000 +0530
 > @@ -2706,7 +2706,11 @@ static void __do_SAK(void *arg)
 >  		}
 >  		task_lock(p);
 >  		if (p->files) {
 > -			rcu_read_lock();
 > +			/*
 > +			 * We don't take a ref to the file, so we must
 > +			 * hold ->file_lock instead.
 > +			 */
 > +			spin_lock(&p->files->file_lock);
 >  			fdt = files_fdtable(p->files);
 >  			for (i=0; i < fdt->max_fds; i++) {
 >  				filp = fcheck_files(p->files, i);
 > @@ -2721,7 +2725,7 @@ static void __do_SAK(void *arg)
 >  					break;
 >  				}
 >  			}
 > -			rcu_read_unlock();
 > +			spin_unlock(&p->files->file_lock);
 >  		}
 >  		task_unlock(p);
 >  	} while_each_task_pid(session, PIDTYPE_SID, p);
 > diff -puN fs/locks.c~fix-proc-fd-ops fs/locks.c
 > --- linux-2.6.16-rcu/fs/locks.c~fix-proc-fd-ops	2006-04-12 21:06:24.000000000 +0530
 > +++ linux-2.6.16-rcu-dipankar/fs/locks.c	2006-04-12 21:06:24.000000000 +0530
 > @@ -2212,7 +2212,12 @@ void steal_locks(fl_owner_t from)
 >  
 >  	lock_kernel();
 >  	j = 0;
 > -	rcu_read_lock();
 > +
 > +	/*
 > +	 * We are not taking a ref to the file structures, so
 > +	 * we need to acquire ->file_lock.
 > +	 */
 > +	spin_lock(&files->file_lock);
 >  	fdt = files_fdtable(files);
 >  	for (;;) {
 >  		unsigned long set;
 > @@ -2230,7 +2235,7 @@ void steal_locks(fl_owner_t from)
 >  			set >>= 1;
 >  		}
 >  	}
 > -	rcu_read_unlock();
 > +	spin_unlock(&files->file_lock);
 >  	unlock_kernel();
 >  }
 >  EXPORT_SYMBOL(steal_locks);
 > diff -puN fs/proc/base.c~fix-proc-fd-ops fs/proc/base.c
 > --- linux-2.6.16-rcu/fs/proc/base.c~fix-proc-fd-ops	2006-04-12 21:06:24.000000000 +0530
 > +++ linux-2.6.16-rcu-dipankar/fs/proc/base.c	2006-04-12 21:06:24.000000000 +0530
 > @@ -294,16 +294,20 @@ static int proc_fd_link(struct inode *in
 >  
 >  	files = get_files_struct(task);
 >  	if (files) {
 > -		rcu_read_lock();
 > +		/*
 > +		 * We are not taking a ref to the file structure, so we must
 > +		 * hold ->file_lock.
 > +		 */
 > +		spin_lock(&files->file_lock);
 >  		file = fcheck_files(files, fd);
 >  		if (file) {
 >  			*mnt = mntget(file->f_vfsmnt);
 >  			*dentry = dget(file->f_dentry);
 > -			rcu_read_unlock();
 > +			spin_unlock(&files->file_lock);
 >  			put_files_struct(files);
 >  			return 0;
 >  		}
 > -		rcu_read_unlock();
 > +		spin_unlock(&files->file_lock);
 >  		put_files_struct(files);
 >  	}
 >  	return -ENOENT;
 > @@ -1485,7 +1489,12 @@ static struct dentry *proc_lookupfd(stru
 >  	if (!files)
 >  		goto out_unlock;
 >  	inode->i_mode = S_IFLNK;
 > -	rcu_read_lock();
 > +
 > +	/*
 > +	 * We are not taking a ref to the file structure, so we must
 > +	 * hold ->file_lock.
 > +	 */
 > +	spin_lock(&files->file_lock);
 >  	file = fcheck_files(files, fd);
 >  	if (!file)
 >  		goto out_unlock2;
 > @@ -1493,7 +1502,7 @@ static struct dentry *proc_lookupfd(stru
 >  		inode->i_mode |= S_IRUSR | S_IXUSR;
 >  	if (file->f_mode & 2)
 >  		inode->i_mode |= S_IWUSR | S_IXUSR;
 > -	rcu_read_unlock();
 > +	spin_unlock(&files->file_lock);
 >  	put_files_struct(files);
 >  	inode->i_op = &proc_pid_link_inode_operations;
 >  	inode->i_size = 64;
 > @@ -1503,7 +1512,7 @@ static struct dentry *proc_lookupfd(stru
 >  	return NULL;
 >  
 >  out_unlock2:
 > -	rcu_read_unlock();
 > +	spin_unlock(&files->file_lock);
 >  	put_files_struct(files);
 >  out_unlock:
 >  	iput(inode);
 > 
 > _
 > 
-
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