Re: [PATCH] xfs: fix race while discarding buffers [V4]

Ben Myers
2012-08-20T20:51:38Z

Hi Carlos,

On Fri, Aug 10, 2012 at 03:01:51PM -0300, Carlos Maiolino wrote:
> While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with
> buffers from lru list), there is a possibility to have xfs_buf_stale() racing
> with it, and removing buffers from dispose list before xfs_buftarg_shrink() does
> it.
>
> This happens because xfs_buftarg_shrink() handle the dispose list without
> locking and the test condition in xfs_buf_stale() checks for the buffer being in
> *any* list:
>
> if (!list_empty(&bp->b_lru)
)

That's cruel and unusual punishment.

> If the buffer happens to be on dispose list, this causes the buffer counter of
> lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink()
> and another in xfs_buf_stale()) causing a wrong account usage of the lru list.
>
> This may cause xfs_buftarg_shrink() to return a wrong value to the memory
> shrinker shrink_slab(), and such account error may also cause an underflowed
> value to be returned; since the counter is lower than the current number of
> items in the lru list, a decrement may happen when the counter is 0, causing
> an underflow on the counter.
>
> The fix uses a new flag field (and a new buffer flag) to serialize buffer
> handling during the shrink process. The new flag field has been designed to use
> btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism.
>
> dchinner, sandeen, aquini and aris also deserve credits for this.
>
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
> fs/xfs/xfs_buf.c | 5 ++++-
> fs/xfs/xfs_buf.h | 41 ++++++++++++++++++++++++-----------------
> 2 files changed, 28 insertions(+), 18 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d7a9dd7..933b793 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -96,6 +96,7 @@ xfs_buf_lru_add(
> atomic_inc(&bp->b_hold);
> list_add_tail(&bp->b_lru, &btp->bt_lru);
> btp->bt_lru_nr++;
> + bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
> }
> spin_unlock(&btp->bt_lru_lock);
> }
> @@ -154,7 +155,8 @@ xfs_buf_stale(
> struct xfs_buftarg *btp = bp->b_target;
>
> spin_lock(&btp->bt_lru_lock);
> - if (!list_empty(&bp->b_lru)) {
> + if (!list_empty(&bp->b_lru) &&
> + !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
> list_del_init(&bp->b_lru);
> btp->bt_lru_nr--;
> atomic_dec(&bp->b_hold);
> @@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
> */
> list_move(&bp->b_lru, &dispose);
> btp->bt_lru_nr--;
> + bp->b_lru_flags |= _XBF_LRU_DISPOSE;
> }
> spin_unlock(&btp->bt_lru_lock);
>
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index d03b73b..7c0b6a0 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -38,27 +38,28 @@ typedef enum {
> XBRW_ZERO = 3, /* Zero target memory */
> } xfs_buf_rw_t;
>
> -#define XBF_READ (1 << 0) /* buffer intended for reading from device */
> -#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
> -#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
> -#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
> -#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
> -#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */
> +#define XBF_READ (1 << 0) /* buffer intended for reading from device */
> +#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
> +#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
> +#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
> +#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
> +#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */
>
> /* I/O hints for the BIO layer */
> -#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
> -#define XBF_FUA (1 << 11)/* force cache write through mode */
> -#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */
> +#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
> +#define XBF_FUA (1 << 11)/* force cache write through mode */
> +#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */
>
> /* flags used only as arguments to access routines */
> -#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
> -#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
> +#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
> +#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
>
> /* flags used only internally */
> -#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
> -#define _XBF_KMEM (1 << 21)/* backed by heap memory */
> -#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
> -#define _XBF_COMPOUND (1 << 23)/* compound buffer */
> +#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
> +#define _XBF_KMEM (1 << 21)/* backed by heap memory */
> +#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
> +#define _XBF_COMPOUND (1 << 23)/* compound buffer */
> +#define _XBF_LRU_DISPOSE (1 << 24)/* buffer being discarded */

It's nice to have them lined up like that.

>
> typedef unsigned int xfs_buf_flags_t;
>
> @@ -72,12 +73,13 @@ typedef unsigned int xfs_buf_flags_t;
> { XBF_SYNCIO, "SYNCIO" }, \
> { XBF_FUA, "FUA" }, \
> { XBF_FLUSH, "FLUSH" }, \
> - { XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
> + { XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\

...and you got rid of an extra space here.

> { XBF_UNMAPPED, "UNMAPPED" }, /* ditto */\
> { _XBF_PAGES, "PAGES" }, \
> { _XBF_KMEM, "KMEM" }, \
> { _XBF_DELWRI_Q, "DELWRI_Q" }, \
> - { _XBF_COMPOUND, "COMPOUND" }
> + { _XBF_COMPOUND, "COMPOUND" }, \
> + { _XBF_LRU_DISPOSE, "LRU_DISPOSE" }
>
> typedef struct xfs_buftarg {
> dev_t bt_dev;
> @@ -124,7 +126,12 @@ typedef struct xfs_buf {
> xfs_buf_flags_t b_flags; /* status flags */
> struct semaphore b_sema; /* semaphore for lockables */
>
> + /*
> + * concurrent access to b_lru and b_lru_flags are protected by
> + * bt_lru_lock and not by b_sema
> + */
> struct list_head b_lru; /* lru list */
> + xfs_buf_flags_t b_lru_flags; /* internal lru status flags */
> wait_queue_head_t b_waiters; /* unpin waiters */
> struct list_head b_list;
> struct xfs_perag *b_pag; /* contains rbtree root */

This looks pretty good to me. Looks like you've been careful about the
locking.

What was the symptom that led to the discovery of this problem?

Reviewed-by: Ben Myers <bpm@sgi.com>

Regards,
Ben