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Remove potential race conditions from access to retriers count #8103

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ywkaras wants to merge 1 commit into from
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Remove potential race conditions from access to retriers count #8103

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8 changes: 3 additions & 5 deletions proxy/ParentConsistentHash.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -235,18 +235,16 @@ ParentConsistentHash::selectParent(bool first_call, ParentResult *result, Reques
Debug("parent_select", "Parent.failedAt = %u, retry = %u, xact_start = %u", static_cast<unsigned>(pRec->failedAt.load()),
static_cast<unsigned>(retry_time), static_cast<unsigned>(request_info->xact_start));
if ((pRec->failedAt.load() + retry_time) < request_info->xact_start) {
if (pRec->retriers.fetch_add(1, std::memory_order_relaxed) < max_retriers) {
if (pRec->inc_retriers(max_retriers)) {
parentRetry = true;
// make sure that the proper state is recorded in the result structure
result->last_parent = pRec->idx;
result->last_lookup = last_lookup;
result->retry = parentRetry;
result->result = PARENT_SPECIFIED;
Debug("parent_select", "Down parent %s is now retryable, retriers = %d, max_retriers = %d", pRec->hostname,
pRec->retriers.load(), max_retriers);
pRec->retriers(), max_retriers);
break;
} else {
pRec->retriers--;
}
}
}
Expand Down Expand Up @@ -395,7 +393,7 @@ ParentConsistentHash::markParentUp(ParentResult *result)

pRec->failedAt = static_cast<time_t>(0);
int old_count = pRec->failCount.exchange(0, std::memory_order_relaxed);
pRec->retriers = 0;
pRec->clear_retriers();

if (old_count > 0) {
Note("http parent proxy %s:%d restored", pRec->hostname, pRec->port);
Expand Down
9 changes: 3 additions & 6 deletions proxy/ParentRoundRobin.cc
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Expand Up @@ -157,18 +157,15 @@ ParentRoundRobin::selectParent(bool first_call, ParentResult *result, RequestDat
} else {
if ((result->wrap_around) ||
(((parents[cur_index].failedAt + retry_time) < request_info->xact_start) && host_stat == TS_HOST_STATUS_UP)) {
if (parents[cur_index].retriers.fetch_add(1, std::memory_order_relaxed) < max_retriers) {
if (parents[cur_index].inc_retriers(max_retriers)) {
Debug("parent_select",
"Parent[%d].failedAt = %u, retry = %u, retriers = %d, max_retriers = %u, xact_start = %" PRId64 " but wrap = %d",
cur_index, static_cast<unsigned>(parents[cur_index].failedAt.load()), retry_time,
parents[cur_index].retriers.load(), max_retriers, static_cast<int64_t>(request_info->xact_start),
result->wrap_around);
cur_index, static_cast<unsigned>(parents[cur_index].failedAt.load()), retry_time, parents[cur_index].retriers(),
max_retriers, static_cast<int64_t>(request_info->xact_start), result->wrap_around);
// Reuse the parent
parentUp = true;
parentRetry = true;
Debug("parent_select", "Parent marked for retry %s:%d", parents[cur_index].hostname, parents[cur_index].port);
} else {
parents[cur_index].retriers--;
}
} else {
parentUp = false;
Expand Down
4 changes: 2 additions & 2 deletions proxy/ParentSelection.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -539,7 +539,7 @@ ParentRecord::ProcessParents(char *val, bool isPrimary)
this->parents[i].name = this->parents[i].hostname;
this->parents[i].available = true;
this->parents[i].weight = weight;
this->parents[i].retriers = 0;
this->parents[i].clear_retriers();
if (tmp3) {
memcpy(this->parents[i].hash_string, tmp3 + 1, strlen(tmp3));
this->parents[i].name = this->parents[i].hash_string;
Expand All @@ -555,7 +555,7 @@ ParentRecord::ProcessParents(char *val, bool isPrimary)
this->secondary_parents[i].name = this->secondary_parents[i].hostname;
this->secondary_parents[i].available = true;
this->secondary_parents[i].weight = weight;
this->secondary_parents[i].retriers = 0;
this->secondary_parents[i].clear_retriers();
if (tmp3) {
memcpy(this->secondary_parents[i].hash_string, tmp3 + 1, strlen(tmp3));
this->secondary_parents[i].name = this->secondary_parents[i].hash_string;
Expand Down
45 changes: 43 additions & 2 deletions proxy/ParentSelection.h
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Original file line number Diff line number Diff line change
Expand Up @@ -109,7 +109,9 @@ struct SimpleRetryResponseCodes {
//
// A record for an individual parent
//
struct pRecord : ATSConsistentHashNode {
class pRecord : public ATSConsistentHashNode
{
public:
char hostname[MAXDNAME + 1];
int port;
std::atomic<time_t> failedAt = 0;
Expand All @@ -119,7 +121,46 @@ struct pRecord : ATSConsistentHashNode {
int idx;
float weight;
char hash_string[MAXDNAME + 1];
std::atomic<int> retriers = 0;

int
retriers() const
{
return _retriers.load(std::memory_order_relaxed);
}

void
clear_retriers()
{
_retriers.store(0, std::memory_order_relaxed);
}

bool
inc_retriers(int max_retriers)
{
ink_assert(max_retriers > 0);

int r = _retriers.load(std::memory_order_relaxed);
while (r < max_retriers) {
if (_retriers.compare_exchange_weak(r, r + 1, std::memory_order_relaxed, std::memory_order_relaxed)) {
return true;
}
}
return false;
}

void
dec_retriers()
{
int r = _retriers.load(std::memory_order_relaxed);
while (r > 0) {
if (_retriers.compare_exchange_weak(r, r - 1, std::memory_order_relaxed, std::memory_order_relaxed)) {
break;
}
}
}

private:
std::atomic<int> _retriers = 0;
};

typedef ControlMatcher<ParentRecord, ParentResult> P_table;
Expand Down
6 changes: 2 additions & 4 deletions proxy/ParentSelectionStrategy.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -67,9 +67,7 @@ ParentSelectionStrategy::markParentDown(ParentResult *result, unsigned int fail_
new_fail_count = pRec->failCount = 1;
} else {
// this was a retry that failed, decrement the retriers count
if ((pRec->retriers--) < 0) {
pRec->retriers = 0;
}
pRec->dec_retriers();
}

Note("Parent %s marked as down %s:%d", (result->retry) ? "retry" : "initially", pRec->hostname, pRec->port);
Expand Down Expand Up @@ -128,7 +126,7 @@ ParentSelectionStrategy::markParentUp(ParentResult *result)
pRec->failedAt = static_cast<time_t>(0);
int old_count = pRec->failCount.exchange(0, std::memory_order_relaxed);
// a retry succeeded, just reset retriers
pRec->retriers = 0;
pRec->clear_retriers();

if (old_count > 0) {
Note("http parent proxy %s:%d restored", pRec->hostname, pRec->port);
Expand Down