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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2] MCE: Fix race condition in mctelem_reserve
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On Wed, 2014-01-22 at 17:17 +0000, Frediano Ziglio wrote:
> From 49b37906afef0981f318064f4cb53a3602bca50a Mon Sep 17 00:00:00 2001
> From: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
> Date: Wed, 22 Jan 2014 10:48:50 +0000
> Subject: [PATCH] MCE: Fix race condition in mctelem_reserve
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> These lines (in mctelem_reserve)
>
> newhead = oldhead->mcte_next;
> if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {
>
> are racy. After you read the newhead pointer it can happen that another
> flow (thread or recursive invocation) change all the list but set head
> with same value. So oldhead is the same as *freelp but you are setting
> a new head that could point to whatever element (even already used).
>
> This patch use instead a bit array and atomic bit operations.
>
> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
> ---
> xen/arch/x86/cpu/mcheck/mctelem.c | 81
> ++++++++++++++-----------------------
> 1 file changed, 30 insertions(+), 51 deletions(-)
>
> Changes from v1:
> - Use bitmap to allow any number of items to be used;
> - Use a single bitmap to simplify reserve loop;
> - Remove HOME flags as was not used anymore.
>
> diff --git a/xen/arch/x86/cpu/mcheck/mctelem.c
> b/xen/arch/x86/cpu/mcheck/mctelem.c
> index 895ce1a..ed8e8d2 100644
> --- a/xen/arch/x86/cpu/mcheck/mctelem.c
> +++ b/xen/arch/x86/cpu/mcheck/mctelem.c
> @@ -37,24 +37,19 @@ struct mctelem_ent {
> void *mcte_data; /* corresponding data payload */
> };
>
> -#define MCTE_F_HOME_URGENT 0x0001U /* free to urgent
> freelist */
> -#define MCTE_F_HOME_NONURGENT 0x0002U /* free to nonurgent
> freelist */
> -#define MCTE_F_CLASS_URGENT 0x0004U /* in use - urgent
> errors */
> -#define MCTE_F_CLASS_NONURGENT 0x0008U /* in use - nonurgent
> errors */
> +#define MCTE_F_CLASS_URGENT 0x0001U /* in use - urgent
> errors */
> +#define MCTE_F_CLASS_NONURGENT 0x0002U /* in use - nonurgent
> errors */
> #define MCTE_F_STATE_FREE 0x0010U /* on a freelist */
> #define MCTE_F_STATE_UNCOMMITTED 0x0020U /* reserved; on no list
> */
> #define MCTE_F_STATE_COMMITTED 0x0040U /* on a committed list
> */
> #define MCTE_F_STATE_PROCESSING 0x0080U /* on a processing list
> */
>
> -#define MCTE_F_MASK_HOME (MCTE_F_HOME_URGENT |
> MCTE_F_HOME_NONURGENT)
> #define MCTE_F_MASK_CLASS (MCTE_F_CLASS_URGENT |
> MCTE_F_CLASS_NONURGENT)
> #define MCTE_F_MASK_STATE (MCTE_F_STATE_FREE | \
> MCTE_F_STATE_UNCOMMITTED | \
> MCTE_F_STATE_COMMITTED | \
> MCTE_F_STATE_PROCESSING)
>
> -#define MCTE_HOME(tep) ((tep)->mcte_flags & MCTE_F_MASK_HOME)
> -
> #define MCTE_CLASS(tep) ((tep)->mcte_flags & MCTE_F_MASK_CLASS)
> #define MCTE_SET_CLASS(tep, new) do { \
> (tep)->mcte_flags &= ~MCTE_F_MASK_CLASS; \
> @@ -69,6 +64,8 @@ struct mctelem_ent {
> #define MC_URGENT_NENT 10
> #define MC_NONURGENT_NENT 20
>
> +#define MC_NENT (MC_URGENT_NENT + MC_NONURGENT_NENT)
> +
> #define MC_NCLASSES (MC_NONURGENT + 1)
>
> #define COOKIE2MCTE(c) ((struct mctelem_ent *)(c))
> @@ -77,11 +74,9 @@ struct mctelem_ent {
> static struct mc_telem_ctl {
> /* Linked lists that thread the array members together.
> *
> - * The free lists are singly-linked via mcte_next, and we allocate
> - * from them by atomically unlinking an element from the head.
> - * Consumed entries are returned to the head of the free list.
> - * When an entry is reserved off the free list it is not linked
> - * on any list until it is committed or dismissed.
> + * The free lists is a bit array where bit 1 means free.
> + * This as element number is quite small and is easy to
> + * atomically allocate that way.
> *
> * The committed list grows at the head and we do not maintain a
> * tail pointer; insertions are performed atomically. The head
> @@ -101,7 +96,7 @@ static struct mc_telem_ctl {
> * we can lock it for updates. The head of the processing list
> * always has the oldest telemetry, and we append (as above)
> * at the tail of the processing list. */
> - struct mctelem_ent *mctc_free[MC_NCLASSES];
> + DECLARE_BITMAP(mctc_free, MC_NENT);
> struct mctelem_ent *mctc_committed[MC_NCLASSES];
> struct mctelem_ent *mctc_processing_head[MC_NCLASSES];
> struct mctelem_ent *mctc_processing_tail[MC_NCLASSES];
> @@ -207,14 +202,14 @@ int mctelem_has_deferred(unsigned int cpu)
> */
> static void mctelem_free(struct mctelem_ent *tep)
> {
> - mctelem_class_t target = MCTE_HOME(tep) == MCTE_F_HOME_URGENT ?
> - MC_URGENT : MC_NONURGENT;
> -
> BUG_ON(tep->mcte_refcnt != 0);
> BUG_ON(MCTE_STATE(tep) != MCTE_F_STATE_FREE);
>
> tep->mcte_prev = NULL;
> - mctelem_xchg_head(&mctctl.mctc_free[target], &tep->mcte_next, tep);
> + tep->mcte_next = NULL;
> +
> + /* set free in array */
> + set_bit(tep - mctctl.mctc_elems, mctctl.mctc_free);
> }
>
> /* Increment the reference count of an entry that is not linked on to
> @@ -274,34 +269,25 @@ void mctelem_init(int reqdatasz)
> }
>
> if ((mctctl.mctc_elems = xmalloc_array(struct mctelem_ent,
> - MC_URGENT_NENT + MC_NONURGENT_NENT)) == NULL ||
> - (datarr = xmalloc_bytes((MC_URGENT_NENT + MC_NONURGENT_NENT) *
> - datasz)) == NULL) {
> + MC_NENT)) == NULL ||
> + (datarr = xmalloc_bytes(MC_NENT * datasz)) == NULL) {
> if (mctctl.mctc_elems)
> xfree(mctctl.mctc_elems);
> printk("Allocations for MCA telemetry failed\n");
> return;
> }
>
> - for (i = 0; i < MC_URGENT_NENT + MC_NONURGENT_NENT; i++) {
> - struct mctelem_ent *tep, **tepp;
> + for (i = 0; i < MC_NENT; i++) {
> + struct mctelem_ent *tep;
>
> tep = mctctl.mctc_elems + i;
> tep->mcte_flags = MCTE_F_STATE_FREE;
> tep->mcte_refcnt = 0;
> tep->mcte_data = datarr + i * datasz;
>
> - if (i < MC_URGENT_NENT) {
> - tepp = &mctctl.mctc_free[MC_URGENT];
> - tep->mcte_flags |= MCTE_F_HOME_URGENT;
> - } else {
> - tepp = &mctctl.mctc_free[MC_NONURGENT];
> - tep->mcte_flags |= MCTE_F_HOME_NONURGENT;
> - }
> -
> - tep->mcte_next = *tepp;
> + __set_bit(i, mctctl.mctc_free);
> + tep->mcte_next = NULL;
> tep->mcte_prev = NULL;
> - *tepp = tep;
> }
> }
>
> @@ -310,32 +296,25 @@ static int mctelem_drop_count;
>
> /* Reserve a telemetry entry, or return NULL if none available.
> * If we return an entry then the caller must subsequently call exactly one
> of
> - * mctelem_unreserve or mctelem_commit for that entry.
> + * mctelem_dismiss or mctelem_commit for that entry.
> */
> mctelem_cookie_t mctelem_reserve(mctelem_class_t which)
> {
> - struct mctelem_ent **freelp;
> - struct mctelem_ent *oldhead, *newhead;
> - mctelem_class_t target = (which == MC_URGENT) ?
> - MC_URGENT : MC_NONURGENT;
> + unsigned bit;
> + unsigned start_bit = (which == MC_URGENT) ? 0 : MC_URGENT_NENT;
>
> - freelp = &mctctl.mctc_free[target];
> for (;;) {
> - if ((oldhead = *freelp) == NULL) {
> - if (which == MC_URGENT && target == MC_URGENT) {
> - /* raid the non-urgent freelist */
> - target = MC_NONURGENT;
> - freelp = &mctctl.mctc_free[target];
> - continue;
> - } else {
> - mctelem_drop_count++;
> - return (NULL);
> - }
> + bit = find_next_bit(mctctl.mctc_free, MC_NENT, start_bit);
> +
> + if (bit >= MC_NENT) {
> + mctelem_drop_count++;
> + return (NULL);
> }
>
> - newhead = oldhead->mcte_next;
> - if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {
> - struct mctelem_ent *tep = oldhead;
> + /* try to allocate, atomically clear free bit */
> + if (test_and_clear_bit(bit, mctctl.mctc_free)) {
> + /* return element we got */
> + struct mctelem_ent *tep = mctctl.mctc_elems + bit;
>
> mctelem_hold(tep);
> MCTE_TRANSITION_STATE(tep, FREE, UNCOMMITTED);
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