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nbd/client: Simplify cookie vs. index computation

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Our code relies on a sentinel cookie value of zero for deciding when a
packet has been handled, as well as relying on array indices between 0
and MAX_NBD_REQUESTS-1 for dereferencing purposes.  As long as we can
symmetrically convert between two forms, there is no reason to go with
the odd choice of using XOR with a random pointer, when we can instead
simplify the mappings with a mere offset of 1.

Using ((uint64_t)-1) as the sentinel instead of NULL such that the two
macros could be entirely eliminated might also be possible, but would
require a more careful audit to find places where we currently rely on
zero-initialization to be interpreted as the sentinel value, so I did
not pursue that course.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230608135653.2918540-7-eblake@redhat.com>
[eblake: enhance commit message]
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
This commit is contained in:
Eric Blake 2023-06-08 08:56:35 -05:00
parent 22efd81104
commit 8cb98a725e
1 changed files with 8 additions and 8 deletions
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16
block/nbd.c
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@ -50,8 +50,8 @@
#define EN_OPTSTR ":exportname="
#define MAX_NBD_REQUESTS 16
#define COOKIE_TO_INDEX(bs, cookie) ((cookie) ^ (uint64_t)(intptr_t)(bs))
#define INDEX_TO_COOKIE(bs, index) ((index) ^ (uint64_t)(intptr_t)(bs))
#define COOKIE_TO_INDEX(cookie) ((cookie) - 1)
#define INDEX_TO_COOKIE(index) ((index) + 1)
typedef struct {
Coroutine *coroutine;
@ -420,7 +420,7 @@ static void coroutine_fn GRAPH_RDLOCK nbd_reconnect_attempt(BDRVNBDState *s)
static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t cookie)
{
int ret;
uint64_t ind = COOKIE_TO_INDEX(s, cookie), ind2;
uint64_t ind = COOKIE_TO_INDEX(cookie), ind2;
QEMU_LOCK_GUARD(&s->receive_mutex);
while (true) {
@ -435,7 +435,7 @@ static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t cookie)
* woken by whoever set s->reply.cookie (or never wait in this
* yield). So, we should not wake it here.
*/
ind2 = COOKIE_TO_INDEX(s, s->reply.cookie);
ind2 = COOKIE_TO_INDEX(s->reply.cookie);
assert(!s->requests[ind2].receiving);
s->requests[ind].receiving = true;
@ -468,7 +468,7 @@ static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t cookie)
nbd_channel_error(s, -EINVAL);
return -EINVAL;
}
ind2 = COOKIE_TO_INDEX(s, s->reply.cookie);
ind2 = COOKIE_TO_INDEX(s->reply.cookie);
if (ind2 >= MAX_NBD_REQUESTS || !s->requests[ind2].coroutine) {
nbd_channel_error(s, -EINVAL);
return -EINVAL;
@ -519,7 +519,7 @@ nbd_co_send_request(BlockDriverState *bs, NBDRequest *request,
qemu_mutex_unlock(&s->requests_lock);
qemu_co_mutex_lock(&s->send_mutex);
request->cookie = INDEX_TO_COOKIE(s, i);
request->cookie = INDEX_TO_COOKIE(i);
assert(s->ioc);
@ -832,7 +832,7 @@ static coroutine_fn int nbd_co_do_receive_one_chunk(
int *request_ret, QEMUIOVector *qiov, void **payload, Error **errp)
{
int ret;
int i = COOKIE_TO_INDEX(s, cookie);
int i = COOKIE_TO_INDEX(cookie);
void *local_payload = NULL;
NBDStructuredReplyChunk *chunk;
@ -1038,7 +1038,7 @@ static bool coroutine_fn nbd_reply_chunk_iter_receive(BDRVNBDState *s,
break_loop:
qemu_mutex_lock(&s->requests_lock);
s->requests[COOKIE_TO_INDEX(s, cookie)].coroutine = NULL;
s->requests[COOKIE_TO_INDEX(cookie)].coroutine = NULL;
s->in_flight--;
qemu_co_queue_next(&s->free_sema);
qemu_mutex_unlock(&s->requests_lock);