Re: [Qemu-devel] [PATCH V15 06/13] quorum: Add quorum mechanism.

["Benoît Canet" <benoit.canet@irqsave.net>]

Le Tuesday 04 Feb 2014 à 16:40:12 (+0100), Kevin Wolf a écrit :
> Am 03.02.2014 um 22:51 hat Benoît Canet geschrieben:
> > From: Benoît Canet <benoit@irqsave.net>
> > 
> > Use gnutls's SHA-256 to compare versions.
> > 
> > Signed-off-by: Benoit Canet <benoit@irqsave.net>
> > ---
> >  block/Makefile.objs       |   2 +-
> >  block/quorum.c            | 386 +++++++++++++++++++++++++++++++++++++++++++++-
> >  configure                 |  36 +++++
> >  docs/qmp/qmp-events.txt   |  33 ++++
> >  include/monitor/monitor.h |   2 +
> >  monitor.c                 |   2 +
> >  6 files changed, 458 insertions(+), 3 deletions(-)
> > 
> > diff --git a/block/Makefile.objs b/block/Makefile.objs
> > index a2650b9..4ca9d43 100644
> > --- a/block/Makefile.objs
> > +++ b/block/Makefile.objs
> > @@ -3,7 +3,7 @@ block-obj-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-c
> >  block-obj-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
> >  block-obj-y += qed-check.o
> >  block-obj-$(CONFIG_VHDX) += vhdx.o vhdx-endian.o vhdx-log.o
> > -block-obj-y += quorum.o
> > +block-obj-$(CONFIG_QUORUM) += quorum.o
> >  block-obj-y += parallels.o blkdebug.o blkverify.o
> >  block-obj-y += snapshot.o qapi.o
> >  block-obj-$(CONFIG_WIN32) += raw-win32.o win32-aio.o
> > diff --git a/block/quorum.c b/block/quorum.c
> > index 699b512..837d261 100644
> > --- a/block/quorum.c
> > +++ b/block/quorum.c
> > @@ -13,7 +13,43 @@
> >   * See the COPYING file in the top-level directory.
> >   */
> >  
> > +#include <gnutls/gnutls.h>
> > +#include <gnutls/crypto.h>
> >  #include "block/block_int.h"
> > +#include "qapi/qmp/qjson.h"
> > +
> > +#define HASH_LENGTH 32
> > +
> > +/* This union holds a vote hash value */
> > +typedef union QuorumVoteValue {
> > +    char h[HASH_LENGTH];       /* SHA-256 hash */
> > +    int64_t l;                 /* simpler 64 bits hash */
> > +} QuorumVoteValue;
> > +
> > +/* A vote item */
> > +typedef struct QuorumVoteItem {
> > +    int index;
> > +    QLIST_ENTRY(QuorumVoteItem) next;
> > +} QuorumVoteItem;
> > +
> > +/* this structure is a vote version. A version is the set of votes sharing the
> > + * same vote value.
> > + * The set of votes will be tracked with the items field and its cardinality is
> > + * vote_count.
> > + */
> > +typedef struct QuorumVoteVersion {
> > +    QuorumVoteValue value;
> > +    int index;
> > +    int vote_count;
> > +    QLIST_HEAD(, QuorumVoteItem) items;
> > +    QLIST_ENTRY(QuorumVoteVersion) next;
> > +} QuorumVoteVersion;
> > +
> > +/* this structure holds a group of vote versions together */
> > +typedef struct QuorumVotes {
> > +    QLIST_HEAD(, QuorumVoteVersion) vote_list;
> > +    int (*compare)(QuorumVoteValue *a, QuorumVoteValue *b);
> > +} QuorumVotes;
> >  
> >  /* the following structure holds the state of one quorum instance */
> >  typedef struct {
> > @@ -60,10 +96,14 @@ struct QuorumAIOCB {
> >      int success_count;          /* number of successfully completed AIOCB */
> >      bool *finished;             /* completion signal for cancel */
> >  
> > +    QuorumVotes votes;
> > +
> >      bool is_read;
> >      int vote_ret;
> >  };
> >  
> > +static void quorum_vote(QuorumAIOCB *acb);
> > +
> >  static void quorum_aio_cancel(BlockDriverAIOCB *blockacb)
> >  {
> >      QuorumAIOCB *acb = container_of(blockacb, QuorumAIOCB, common);
> > @@ -81,10 +121,12 @@ static AIOCBInfo quorum_aiocb_info = {
> >      .cancel             = quorum_aio_cancel,
> >  };
> >  
> > +static int quorum_vote_error(QuorumAIOCB *acb);
> > +
> 
> What's the reason for putting the forward declaration here? This is
> neither directly before the first user nor at the top.
> 
> In fact, the next occurence of quorum_vote_error() is the implementation
> of the function, so the forward declaration is completely unnecessary.
> 
> >  static void quorum_aio_finalize(QuorumAIOCB *acb)
> >  {
> >      BDRVQuorumState *s = acb->bqs;
> > -    int ret = 0;
> > +    int i, ret = 0;
> >  
> >      for (i = 0; i < s->total; i++) {
> >          qemu_vfree(acb->aios[i].buf);
> > @@ -92,6 +134,10 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
> >          acb->aios[i].ret = 0;
> >      }
> >  
> > +    if (acb->vote_ret) {
> > +        ret = acb->vote_ret;
> > +    }
> > +
> >      acb->common.cb(acb->common.opaque, ret);
> >      if (acb->finished) {
> >          *acb->finished = true;
> > @@ -103,6 +149,27 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
> >      qemu_aio_release(acb);
> >  }
> >  
> > +static int quorum_sha256_compare(QuorumVoteValue *a, QuorumVoteValue *b)
> > +{
> > +    return memcmp(a->h, b->h, HASH_LENGTH);
> > +}
> > +
> > +static int quorum_64bits_compare(QuorumVoteValue *a, QuorumVoteValue *b)
> > +{
> > +    int64_t i = a->l;
> > +    int64_t j = b->l;
> > +
> > +    if (i < j) {
> > +        return -1;
> > +    }
> > +
> > +    if (i > j) {
> > +        return 1;
> > +    }
> > +
> > +    return 0;
> > +}
> 
> The usual way to implement this is 'return a->l - b->l;', because if you
> expect memcmp() to return a valid value for the compare function you
> can't assume that it's normalised to -1/0/1 anyway.
> 
> As you only ever use the result as a bool, you could alternatively
> even declare the function as such and do 'return a->l != b->l;'.
> 
> >  static QuorumAIOCB *quorum_aio_get(BDRVQuorumState *s,
> >                                     BlockDriverState *bs,
> >                                     QEMUIOVector *qiov,
> > @@ -122,6 +189,7 @@ static QuorumAIOCB *quorum_aio_get(BDRVQuorumState *s,
> >      acb->count = 0;
> >      acb->success_count = 0;
> >      acb->finished = NULL;
> > +    acb->votes.compare = quorum_sha256_compare;
> >      acb->is_read = false;
> >      acb->vote_ret = 0;
> >  
> 
> You need to initialise votes.vote_list as well.
> 
> > @@ -151,9 +219,323 @@ static void quorum_aio_cb(void *opaque, int ret)
> >          return;
> >      }
> >  
> > +    /* Do the vote on read */
> > +    if (acb->is_read) {
> > +        quorum_vote(acb);
> > +    }
> > +
> >      quorum_aio_finalize(acb);
> >  }
> >  
> > +static void quorum_report_bad(QuorumAIOCB *acb, char *node_name)
> > +{
> > +    QObject *data;
> > +    data = qobject_from_jsonf("{ 'node-name': \"%s\""
> > +                              ", 'sector-num': %" PRId64
> > +                              ", 'sectors-count': %i }",
> > +                              node_name,
> 
> Can't node_name be NULL here?

No node_name is a member of BlockDriverState so the only thing that could happen
is bs->node_name[0] == '\0'
Yet I will add a security.

> 
> > +                              acb->sector_num,
> > +                              acb->nb_sectors);
> > +    monitor_protocol_event(QEVENT_QUORUM_REPORT_BAD, data);
> > +    qobject_decref(data);
> > +}
> > +
> > +static void quorum_report_failure(QuorumAIOCB *acb)
> > +{
> > +    QObject *data;
> > +    data = qobject_from_jsonf("{ 'sector-num': %" PRId64
> > +                              ", 'sectors-count': %i }",
> > +                              acb->sector_num,
> > +                              acb->nb_sectors);
> 
> If I have multiple quorum devices, this event doesn't tell me, which one
> it is about.
> 
> > +    monitor_protocol_event(QEVENT_QUORUM_FAILURE, data);
> > +    qobject_decref(data);
> > +}
> > +
> > +static void quorum_report_bad_versions(BDRVQuorumState *s,
> > +                                       QuorumAIOCB *acb,
> > +                                       QuorumVoteValue *value)
> > +{
> > +    QuorumVoteVersion *version;
> > +    QuorumVoteItem *item;
> > +
> > +    QLIST_FOREACH(version, &acb->votes.vote_list, next) {
> > +        if (!acb->votes.compare(&version->value, value)) {
> > +            continue;
> > +        }
> > +        QLIST_FOREACH(item, &version->items, next) {
> > +            quorum_report_bad(acb, s->bs[item->index]->node_name);
> > +        }
> > +    }
> > +}
> > +
> > +static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> > +{
> > +    int i;
> > +    assert(dest->niov == source->niov);
> > +    assert(dest->size == source->size);
> > +    for (i = 0; i < source->niov; i++) {
> > +        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> > +        memcpy(dest->iov[i].iov_base,
> > +               source->iov[i].iov_base,
> > +               source->iov[i].iov_len);
> > +    }
> > +}
> > +
> > +static void quorum_count_vote(QuorumVotes *votes,
> > +                              QuorumVoteValue *value,
> > +                              int index)
> > +{
> > +    QuorumVoteVersion *v = NULL, *version = NULL;
> > +    QuorumVoteItem *item;
> > +
> > +    /* look if we have something with this hash */
> > +    QLIST_FOREACH(v, &votes->vote_list, next) {
> > +        if (!votes->compare(&v->value, value)) {
> > +            version = v;
> > +            break;
> > +        }
> > +    }
> > +
> > +    /* It's a version not yet in the list add it */
> > +    if (!version) {
> > +        version = g_new0(QuorumVoteVersion, 1);
> > +        QLIST_INIT(&version->items);
> > +        memcpy(&version->value, value, sizeof(version->value));
> > +        version->index = index;
> > +        version->vote_count = 0;
> > +        QLIST_INSERT_HEAD(&votes->vote_list, version, next);
> > +    }
> > +
> > +    version->vote_count++;
> > +
> > +    item = g_new0(QuorumVoteItem, 1);
> > +    item->index = index;
> > +    QLIST_INSERT_HEAD(&version->items, item, next);
> > +}
> > +
> > +static void quorum_free_vote_list(QuorumVotes *votes)
> > +{
> > +    QuorumVoteVersion *version, *next_version;
> > +    QuorumVoteItem *item, *next_item;
> > +
> > +    QLIST_FOREACH_SAFE(version, &votes->vote_list, next, next_version) {
> > +        QLIST_REMOVE(version, next);
> > +        QLIST_FOREACH_SAFE(item, &version->items, next, next_item) {
> > +            QLIST_REMOVE(item, next);
> > +            g_free(item);
> > +        }
> > +        g_free(version);
> > +    }
> > +}
> > +
> > +static int quorum_compute_hash(QuorumAIOCB *acb, int i, QuorumVoteValue *hash)
> > +{
> > +    int j, ret;
> > +    gnutls_hash_hd_t dig;
> > +    QEMUIOVector *qiov = &acb->aios[i].qiov;
> > +
> > +    ret = gnutls_hash_init(&dig, GNUTLS_DIG_SHA256);
> > +
> > +    if (ret < 0) {
> > +        return ret;
> > +    }
> > +
> > +    for (j = 0; j < qiov->niov; j++) {
> > +        ret = gnutls_hash(dig, qiov->iov[j].iov_base, qiov->iov[j].iov_len);
> > +        if (ret < 0) {
> > +            break;
> > +        }
> > +    }
> > +
> > +    gnutls_hash_deinit(dig, (void *) hash);
> > +    return ret;
> > +}
> > +
> > +static QuorumVoteVersion *quorum_get_vote_winner(QuorumVotes *votes)
> > +{
> > +    int i = 0;
> 
> I like obvious variable names. This must be a loop counter.
> 
> > +    QuorumVoteVersion *candidate, *winner = NULL;
> > +
> > +    QLIST_FOREACH(candidate, &votes->vote_list, next) {
> > +        if (candidate->vote_count > i) {
> > +            i = candidate->vote_count;
> 
> Wait, what? This doesn't quite look like a loop.
> 
> > +            winner = candidate;
> > +        }
> > +    }
> > +
> > +    return winner;
> > +}
> > +
> > +static bool quorum_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
> > +{
> > +    int i;
> > +    int result;
> > +
> > +    assert(a->niov == b->niov);
> > +    for (i = 0; i < a->niov; i++) {
> > +        assert(a->iov[i].iov_len == b->iov[i].iov_len);
> > +        result = memcmp(a->iov[i].iov_base,
> > +                        b->iov[i].iov_base,
> > +                        a->iov[i].iov_len);
> > +        if (result) {
> > +            return false;
> > +        }
> > +    }
> > +
> > +    return true;
> > +}
> 
> I thought we introduced qemu_iovec_compare() earlier in this series to
> do exactly this, except more generically?
> 
> I see that you call one or the other depending on whether we're running
> in blkverify mode, but what is the difference in the semantics? Either
> both are the same and there is no reason to have both, or one of them
> must have non-obvious semantics and lacks proper documentation.
> 
> > +static void GCC_FMT_ATTR(2, 3) quorum_err(QuorumAIOCB *acb,
> > +                                          const char *fmt, ...)
> > +{
> > +    va_list ap;
> > +
> > +    va_start(ap, fmt);
> > +    fprintf(stderr, "quorum: sector_num=%" PRId64 " nb_sectors=%d ",
> > +            acb->sector_num, acb->nb_sectors);
> > +    vfprintf(stderr, fmt, ap);
> > +    fprintf(stderr, "\n");
> > +    va_end(ap);
> > +    exit(1);
> > +}
> > +
> > +static bool quorum_compare(QuorumAIOCB *acb,
> > +                           QEMUIOVector *a,
> > +                           QEMUIOVector *b)
> > +{
> > +    BDRVQuorumState *s = acb->bqs;
> > +    ssize_t offset;
> > +
> > +    /* This driver will replace blkverify in this particular case */
> > +    if (s->is_blkverify) {
> > +        offset = qemu_iovec_compare(a, b);
> > +        if (offset != -1) {
> > +            quorum_err(acb, "contents mismatch in sector %" PRId64,
> > +                       acb->sector_num +
> > +                       (uint64_t)(offset / BDRV_SECTOR_SIZE));
> > +        }
> > +        return true;
> > +    }
> > +
> > +    return quorum_iovec_compare(a, b);
> > +}
> > +
> > +/* Do a vote to get the error code */
> > +static int quorum_vote_error(QuorumAIOCB *acb)
> > +{
> > +    BDRVQuorumState *s = acb->bqs;
> > +    QuorumVoteVersion *winner = NULL;
> > +    QuorumVotes error_votes;
> > +    QuorumVoteValue result_value;
> > +    int i, ret = 0;
> > +    bool error = false;
> > +
> > +    QLIST_INIT(&error_votes.vote_list);
> > +    error_votes.compare = quorum_64bits_compare;
> > +
> > +    for (i = 0; i < s->total; i++) {
> > +        ret = acb->aios[i].ret;
> > +        if (ret) {
> > +            error = true;
> > +            result_value.l = ret;
> > +            quorum_count_vote(&error_votes, &result_value, i);
> > +        }
> > +    }
> > +
> > +    if (error) {
> > +        winner = quorum_get_vote_winner(&error_votes);
> > +        ret = winner->value.l;
> > +    }
> > +
> > +    quorum_free_vote_list(&error_votes);
> > +
> > +    return ret;
> > +}
> > +
> > +static void quorum_vote(QuorumAIOCB *acb)
> > +{
> > +    bool quorum = false;
> > +    int i, j, ret;
> > +    QuorumVoteValue hash;
> > +    BDRVQuorumState *s = acb->bqs;
> > +    QuorumVoteVersion *winner;
> > +
> > +    QLIST_INIT(&acb->votes.vote_list);
> > +
> > +    /* if we don't get enough successful read use the first error code */
> > +    if (acb->success_count < s->threshold) {
> > +        acb->vote_ret = quorum_vote_error(acb);
> > +        quorum_report_failure(acb);
> > +        return;
> > +    }
> > +
> > +    /* get the index of the first successful read (we are sure to find one) */
> > +    for (i = 0; i < s->total; i++) {
> > +        if (!acb->aios[i].ret) {
> > +            break;
> > +        }
> > +    }
> 
> "we are sure to find one" is spelt "assert(i < s->total);"
> 
> > +
> > +    /* compare this read with all other successful read looking for quorum */
> > +    for (j = i + 1; j < s->total; j++) {
> > +        if (acb->aios[j].ret) {
> > +            continue;
> > +        }
> > +        quorum = quorum_compare(acb, &acb->aios[i].qiov, &acb->aios[j].qiov);
> > +        if (!quorum) {
> > +            break;
> > +       }
> > +    }
> > +
> > +    /* Every successful read agrees and their count is higher or equal threshold
> > +     * -> Quorum
> > +     */
> > +    if (quorum && acb->success_count >= s->threshold) {
> > +        quorum_copy_qiov(acb->qiov, &acb->aios[i].qiov);
> > +        return;
> > +    }
> 
> For threshold == success_count == 1, the condition in the comment is
> fulfilled, but the one in the code isn't.
> 
> > +
> > +    /* compute hashs for each successful read, also store indexes */
> > +    for (i = 0; i < s->total; i++) {
> > +        if (acb->aios[i].ret) {
> > +            continue;
> > +        }
> > +        ret = quorum_compute_hash(acb, i, &hash);
> > +        /* if ever the hash computation failed */
> > +        if (ret < 0) {
> > +            acb->vote_ret = ret;
> > +            goto free_exit;
> > +        }
> > +        quorum_count_vote(&acb->votes, &hash, i);
> > +    }
> > +
> > +    /* vote to select the most represented version */
> > +    winner = quorum_get_vote_winner(&acb->votes);
> > +    /* every vote version are differents -> error */
> > +    if (!winner) {
> 
> Can this happen? This means that there was no vote at all.
> 
> > +        quorum_report_failure(acb);
> > +        acb->vote_ret = -EIO;
> > +        goto free_exit;
> > +    }
> > +
> > +    /* if the winner count is smaller than threshold the read fails */
> > +    if (winner->vote_count < s->threshold) {
> > +        quorum_report_failure(acb);
> > +        acb->vote_ret = -EIO;
> > +        goto free_exit;
> > +    }
> > +
> > +    /* we have a winner: copy it */
> > +    quorum_copy_qiov(acb->qiov, &acb->aios[winner->index].qiov);
> > +
> > +    /* some versions are bad print them */
> > +    quorum_report_bad_versions(s, acb, &winner->value);
> > +
> > +free_exit:
> > +    /* free lists */
> > +    quorum_free_vote_list(&acb->votes);
> > +}
> > +
> >  static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> >                                           int64_t sector_num,
> >                                           QEMUIOVector *qiov,
> > @@ -175,7 +557,7 @@ static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> >      }
> >  
> >      for (i = 0; i < s->total; i++) {
> > -        bdrv_aio_readv(s->bs[i], sector_num, qiov, nb_sectors,
> > +        bdrv_aio_readv(s->bs[i], sector_num, &acb->aios[i].qiov, nb_sectors,
> >                         quorum_aio_cb, &acb->aios[i]);
> >      }
> 
> Why don't you do this from the beginning?
> 
> Kevin