Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

Al Viro
2010-01-24T19:53:11Z

On Sun, Jan 24, 2010 at 02:41:15PM +0300, Dmitry Monakhov wrote:
> Currently on rw=>ro remount we have following race
> | mount /mnt -oremount,ro | write-task |
> |-------------------------+------------|
> | | open(RDWR) |
> | shrink_dcache_sb(sb); | |
> | sync_filesystem(sb); | |
> | | write() |
> | | close() |
> | fs_may_remount_ro(sb) | |
> | sb->s_flags = new_flags | |
> Later writeback or sync() will result in error due to MS_RDONLY flag
> In case of ext4 this result in jbd2_start failure on writeback
> ext4_da_writepages: jbd2_start: 1024 pages, ino 1431; err -30
> In fact all others are affected by this error but it is not visible
> because the skip s_flags check on writeback. For example ext3 check
> (s_flags & MS_RDONLY) only if page has no buffers during journal start.
>
> In order to prevent the race we have to block new writers before
> fs_may_remount_ro() and sync_filesystem(). Let's introduce new
> sb->s_flags MS_RO_REMOUNT flag for this purpose. But suddenly we have
> no available space in MS_XXX bits, let's share this bit with MS_REMOUNT.
> This is possible because MS_REMOUNT used only for passing arguments
> from flags to sys_mount() and never used in sb->s_flags.

It's not a solution. You get an _attempted_ remount ro making writes
fail, even if it's going to be unsuccessful. No go...