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    Subject[52/90] fix shrink_dcache_parent() livelock
    3.0-stable review patch.  If anyone has any objections, please let me know.


    From: Miklos Szeredi <>

    commit eaf5f9073533cde21c7121c136f1c3f072d9cf59 upstream.

    Two (or more) concurrent calls of shrink_dcache_parent() on the same dentry may
    cause shrink_dcache_parent() to loop forever.

    Here's what appears to happen:

    1 - CPU0: select_parent(P) finds C and puts it on dispose list, returns 1

    2 - CPU1: select_parent(P) locks P->d_lock

    3 - CPU0: shrink_dentry_list() locks C->d_lock
    dentry_kill(C) tries to lock P->d_lock but fails, unlocks C->d_lock

    4 - CPU1: select_parent(P) locks C->d_lock,
    moves C from dispose list being processed on CPU0 to the new
    dispose list, returns 1

    5 - CPU0: shrink_dentry_list() finds dispose list empty, returns

    6 - Goto 2 with CPU0 and CPU1 switched

    Basically select_parent() steals the dentry from shrink_dentry_list() and thinks
    it found a new one, causing shrink_dentry_list() to think it's making progress
    and loop over and over.

    One way to trigger this is to make udev calls stat() on the sysfs file while it
    is going away.

    Having a file in /lib/udev/rules.d/ with only this one rule seems to the trick:

    ATTR{vendor}=="0x8086", ATTR{device}=="0x10ca", ENV{PCI_SLOT_NAME}="%k", ENV{MATCHADDR}="$attr{address}", RUN+="/bin/true"

    Then execute the following loop:

    while true; do
    echo -bond0 > /sys/class/net/bonding_masters
    echo +bond0 > /sys/class/net/bonding_masters
    echo -bond1 > /sys/class/net/bonding_masters
    echo +bond1 > /sys/class/net/bonding_masters

    One fix would be to check all callers and prevent concurrent calls to
    shrink_dcache_parent(). But I think a better solution is to stop the
    stealing behavior.

    This patch adds a new dentry flag that is set when the dentry is added to the
    dispose list. The flag is cleared in dentry_lru_del() in case the dentry gets a
    new reference just before being pruned.

    If the dentry has this flag, select_parent() will skip it and let
    shrink_dentry_list() retry pruning it. With select_parent() skipping those
    dentries there will not be the appearance of progress (new dentries found) when
    there is none, hence shrink_dcache_parent() will not loop forever.

    Set the flag is also set in prune_dcache_sb() for consistency as suggested by

    Signed-off-by: Miklos Szeredi <>
    Signed-off-by: Al Viro <>
    Signed-off-by: Greg Kroah-Hartman <>

    fs/dcache.c | 14 ++++++++++----
    include/linux/dcache.h | 1 +
    2 files changed, 11 insertions(+), 4 deletions(-)

    --- a/fs/dcache.c
    +++ b/fs/dcache.c
    @@ -241,6 +241,7 @@ static void dentry_lru_add(struct dentry
    static void __dentry_lru_del(struct dentry *dentry)
    + dentry->d_flags &= ~DCACHE_SHRINK_LIST;
    @@ -753,6 +754,7 @@ relock:
    } else {
    list_move_tail(&dentry->d_lru, &tmp);
    + dentry->d_flags |= DCACHE_SHRINK_LIST;
    if (!--cnt)
    @@ -1144,14 +1146,18 @@ resume:
    * move only zero ref count dentries to the end
    * of the unused list for prune_dcache
    + *
    + * Those which are presently on the shrink list, being processed
    + * by shrink_dentry_list(), shouldn't be moved. Otherwise the
    + * loop in shrink_dcache_parent() might not make any progress
    + * and loop forever.
    - if (!dentry->d_count) {
    + if (dentry->d_count) {
    + dentry_lru_del(dentry);
    + } else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
    - } else {
    - dentry_lru_del(dentry);
    * We can return to the caller if we have found some (this
    * ensures forward progress). We'll be coming back to find
    --- a/include/linux/dcache.h
    +++ b/include/linux/dcache.h
    @@ -207,6 +207,7 @@ struct dentry_operations {

    #define DCACHE_CANT_MOUNT 0x0100
    #define DCACHE_GENOCIDE 0x0200
    +#define DCACHE_SHRINK_LIST 0x0400

    #define DCACHE_OP_HASH 0x1000
    #define DCACHE_OP_COMPARE 0x2000

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      Last update: 2012-01-24 00:57    [W:0.025 / U:11.376 seconds]
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