fs: Fix hang with BSD accounting on frozen filesystem

When BSD process accounting is enabled and logs information to a
filesystem which gets frozen, system easily becomes unusable because
each attempt to account process information blocks. Thus e.g. every task
gets blocked in exit.

It seems better to drop accounting information (which can already happen
when filesystem is running out of space) instead of locking system up.
So we just skip the write if the filesystem is frozen.
Reported-by: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2227,6 +2227,13 @@ static inline void file_start_write(struct file *file)
 	__sb_start_write(file_inode(file)->i_sb, SB_FREEZE_WRITE, true);
 }
 
+static inline bool file_start_write_trylock(struct file *file)
+{
+	if (!S_ISREG(file_inode(file)->i_mode))
+		return true;
+	return __sb_start_write(file_inode(file)->i_sb, SB_FREEZE_WRITE, false);
+}
+
 static inline void file_end_write(struct file *file)
 {
 	if (!S_ISREG(file_inode(file)->i_mode))

kernel/acct.c

@@ -539,11 +539,16 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 	ac.ac_rw = encode_comp_t(ac.ac_io / 1024);
 	ac.ac_swaps = encode_comp_t(0);
 
+	/*
+	 * Get freeze protection. If the fs is frozen, just skip the write
+	 * as we could deadlock the system otherwise.
+	 */
+	if (!file_start_write_trylock(file))
+		goto out;
 	/*
 	 * Kernel segment override to datasegment and write it
 	 * to the accounting file.
 	 */
-	file_start_write(file);
 	fs = get_fs();
 	set_fs(KERNEL_DS);
 	/*