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    Subject[PATCH] sys_swapon: fix inode locking
    Date
    A conflict between 52c50567d8ab0a0a87f12cceaa4194967854f0bd (mm: swap:
    unlock swapfile inode mutex before closing file on bad swapfiles) and
    83ef99befc324803a54cf2a5fab5a322df3a99d6 (sys_swapon: remove did_down
    variable) caused a double unlock of the inode mutex (once in bad_swap:
    before the filp_close, once at the end just before returning).

    The patch which added the extra unlock cleared did_down to avoid
    unlocking twice, but the other patch removed the did_down variable.

    To fix, set inode to NULL after the first unlock, since it will be used
    after that point only for the final unlock.

    While checking this patch, I found a path which could unlock without
    locking, in case the same inode was added as a swapfile twice. To fix,
    move the setting of the inode variable further down, to just before
    claim_swapfile, which will lock the inode before doing anything else.

    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Eric B Munson <emunson@mgebm.net>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
    ---
    mm/swapfile.c | 7 +++++--
    1 files changed, 5 insertions(+), 2 deletions(-)

    diff --git a/mm/swapfile.c b/mm/swapfile.c
    index aafcf36..71b42ec 100644
    --- a/mm/swapfile.c
    +++ b/mm/swapfile.c
    @@ -2088,7 +2088,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)

    p->swap_file = swap_file;
    mapping = swap_file->f_mapping;
    - inode = mapping->host;

    for (i = 0; i < nr_swapfiles; i++) {
    struct swap_info_struct *q = swap_info[i];
    @@ -2101,6 +2100,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
    }
    }

    + inode = mapping->host;
    + /* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
    error = claim_swapfile(p, inode);
    if (unlikely(error))
    goto bad_swap;
    @@ -2187,8 +2188,10 @@ bad_swap:
    spin_unlock(&swap_lock);
    vfree(swap_map);
    if (swap_file) {
    - if (inode && S_ISREG(inode->i_mode))
    + if (inode && S_ISREG(inode->i_mode)) {
    mutex_unlock(&inode->i_mutex);
    + inode = NULL;
    + }
    filp_close(swap_file, NULL);
    }
    out:
    --
    1.7.4


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