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Date: Wed, 2 Apr 2014 13:47:07 -0400 From: Theodore Ts'o <tytso@....edu> To: Fabian Frederick <fabf@...net.be> Cc: linux-kernel <linux-kernel@...r.kernel.org>, akpm <akpm@...ux-foundation.org>, jfs-discussion@...ts.sourceforge.net Subject: Re: [PATCH 1/1] fs/jfs/jfs_inode.c: atomically set inode->i_flags On Wed, Apr 02, 2014 at 07:29:50PM +0200, Fabian Frederick wrote: > + set_mask_bits(&inode->i_flags, S_IMMUTABLE | S_APPEND | S_NOATIME | > + S_DIRSYNC | S_SYNC, new_fl); Warning --- per discussion with Linus over the weekend, he decided he really didn't like the set_mask_bits() interface, because it was too prone to abuse. Unfortuantely, Linus had already included an earlier version of my patch that used set_mask_bits() in 3.14 without letting me know. This is what I have in my ext4 tree for the merge window, which is undergoing final testing at the moment. This will cause a patch conflict, and it's likely (given Linus's dislike of the set_mask_bits interface) that set_mask_bits() will end up getting removed from the tree, if not when he fixes up the merge conflict, but subsequent to -rc1. - Ted ext4: atomically set inode->i_flags in ext4_set_inode_flags() Use cmpxchg() to atomically set i_flags instead of clearing out the S_IMMUTABLE, S_APPEND, etc. flags and then setting them from the EXT4_IMMUTABLE_FL, EXT4_APPEND_FL flags, since this opens up a race where an immutable file has the immutable flag cleared for a brief window of time. Reported-by: John Sullivan <jsrhbz@...argh.force9.co.uk> Signed-off-by: "Theodore Ts'o" <tytso@....edu> Cc: stable@...nel.org --- fs/ext4/inode.c | 14 ++++++++------ fs/inode.c | 31 +++++++++++++++++++++++++++++++ include/linux/fs.h | 3 +++ 3 files changed, 42 insertions(+), 6 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index b5e182a..df067c3 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -3938,18 +3938,20 @@ int ext4_get_inode_loc(struct inode *inode, struct ext4_iloc *iloc) void ext4_set_inode_flags(struct inode *inode) { unsigned int flags = EXT4_I(inode)->i_flags; + unsigned int new_fl = 0; - inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC); if (flags & EXT4_SYNC_FL) - inode->i_flags |= S_SYNC; + new_fl |= S_SYNC; if (flags & EXT4_APPEND_FL) - inode->i_flags |= S_APPEND; + new_fl |= S_APPEND; if (flags & EXT4_IMMUTABLE_FL) - inode->i_flags |= S_IMMUTABLE; + new_fl |= S_IMMUTABLE; if (flags & EXT4_NOATIME_FL) - inode->i_flags |= S_NOATIME; + new_fl |= S_NOATIME; if (flags & EXT4_DIRSYNC_FL) - inode->i_flags |= S_DIRSYNC; + new_fl |= S_DIRSYNC; + inode_set_flags(inode, new_fl, + S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC); } /* Propagate flags from i_flags to EXT4_I(inode)->i_flags */ diff --git a/fs/inode.c b/fs/inode.c index 4bcdad3..26f95ce 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -1899,3 +1899,34 @@ void inode_dio_done(struct inode *inode) wake_up_bit(&inode->i_state, __I_DIO_WAKEUP); } EXPORT_SYMBOL(inode_dio_done); + +/* + * inode_set_flags - atomically set some inode flags + * + * Note: the caller should be holding i_mutex, or else be sure that + * they have exclusive access to the inode structure (i.e., while the + * inode is being instantiated). The reason for the cmpxchg() loop + * --- which wouldn't be necessary if all code paths which modify + * i_flags actually followed this rule, is that there is at least one + * code path which doesn't today --- for example, + * __generic_file_aio_write() calls file_remove_suid() without holding + * i_mutex --- so we use cmpxchg() out of an abundance of caution. + * + * In the long run, i_mutex is overkill, and we should probably look + * at using the i_lock spinlock to protect i_flags, and then make sure + * it is so documented in include/linux/fs.h and that all code follows + * the locking convention!! + */ +void inode_set_flags(struct inode *inode, unsigned int flags, + unsigned int mask) +{ + unsigned int old_flags, new_flags; + + WARN_ON_ONCE(flags & ~mask); + do { + old_flags = ACCESS_ONCE(inode->i_flags); + new_flags = (old_flags & ~mask) | flags; + } while (unlikely(cmpxchg(&inode->i_flags, old_flags, + new_flags) != old_flags)); +} +EXPORT_SYMBOL(inode_set_flags); diff --git a/include/linux/fs.h b/include/linux/fs.h index 6082956..5d1f6fa 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2556,6 +2556,9 @@ static inline ssize_t blockdev_direct_IO(int rw, struct kiocb *iocb, void inode_dio_wait(struct inode *inode); void inode_dio_done(struct inode *inode); +extern void inode_set_flags(struct inode *inode, unsigned int flags, + unsigned int mask); + extern const struct file_operations generic_ro_fops; #define special_file(m) (S_ISCHR(m)||S_ISBLK(m)||S_ISFIFO(m)||S_ISSOCK(m)) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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