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rgw: fix lock scope in ObjectCache::get()

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in the touch_lru() case, we promote the shared_lock to a unique_lock.
but because the unique_lock is in a nested scope, the lock drops with
its scope and we continue accessing the map without any protection

this moves the unique_lock up to function scope, where it's
constructed as unlocked with std::defer_lock. after promotion, this
lock will be held until the function returns

Fixes: https://tracker.ceph.com/issues/52800

Signed-off-by: Casey Bodley <cbodley@redhat.com>
This commit is contained in:
Casey Bodley 2021-10-18 11:23:49 -04:00
parent 19535b1d0e
commit 1aee987ec8
1 changed files with 3 additions and 2 deletions
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5
src/rgw/rgw_cache.cc
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@ -14,6 +14,7 @@ int ObjectCache::get(const DoutPrefixProvider *dpp, const string& name, ObjectCa
{
std::shared_lock rl{lock};
std::unique_lock wl{lock, std::defer_lock}; // may be promoted to write lock
if (!enabled) {
return -ENOENT;
}
@ -30,7 +31,7 @@ int ObjectCache::get(const DoutPrefixProvider *dpp, const string& name, ObjectCa
(ceph::coarse_mono_clock::now() - iter->second.info.time_added) > expiry) {
ldpp_dout(dpp, 10) << "cache get: name=" << name << " : expiry miss" << dendl;
rl.unlock();
std::unique_lock wl{lock}; // write lock for insertion
wl.lock(); // write lock for expiration
// check that wasn't already removed by other thread
iter = cache_map.find(name);
if (iter != cache_map.end()) {
@ -51,7 +52,7 @@ int ObjectCache::get(const DoutPrefixProvider *dpp, const string& name, ObjectCa
ldpp_dout(dpp, 20) << "cache get: touching lru, lru_counter=" << lru_counter
<< " promotion_ts=" << entry->lru_promotion_ts << dendl;
rl.unlock();
std::unique_lock wl{lock}; // write lock for insertion
wl.lock(); // write lock for touch_lru()
/* need to redo this because entry might have dropped off the cache */
iter = cache_map.find(name);
if (iter == cache_map.end()) {