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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3DC5E21163 X-Spam-Score: -3.01 X-Rspamd-Action: no action X-Spamd-Result: default: False [-3.01 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[37]; FUZZY_BLOCKED(0.00)[rspamd.com]; MIME_TRACE(0.00)[0:+]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FREEMAIL_ENVRCPT(0.00)[gmail.com,sina.com]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; MID_RHS_MATCH_FROM(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[kernel.org,gmail.com,arm.com,linux-foundation.org,infradead.org,oracle.com,suse.com,cmpxchg.org,intel.com,techsingularity.net,redhat.com,stgolabs.net,sina.com,google.com,linux.dev,soleen.com,lwn.net,vger.kernel.org,kvack.org,android.com]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_VIA_SMTP_AUTH(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; TAGGED_RCPT(0.00)[]; R_RATELIMIT(0.00)[to_ip_from(RLk41rrgs15z4i1nmqiwtynpyh)]; ASN(0.00)[asn:25478, ipnet:::/0, country:RU]; DBL_BLOCKED_OPENRESOLVER(0.00)[infradead.org:email,suse.cz:dkim,suse.cz:mid,suse.cz:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO X-Spam-Level: On 2/6/25 03:52, Suren Baghdasaryan wrote: > For speculative lookups where a successful inc_not_zero() pins the > object, but where we still need to double check if the object acquired > is indeed the one we set out to acquire (identity check), needs this > validation to happen *after* the increment. > Similarly, when a new object is initialized and its memory might have > been previously occupied by another object, all stores to initialize the > object should happen *before* refcount initialization. > > Notably SLAB_TYPESAFE_BY_RCU is one such an example when this ordering > is required for reference counting. > > Add refcount_{add|inc}_not_zero_acquire() to guarantee the proper ordering > between acquiring a reference count on an object and performing the > identity check for that object. > Add refcount_set_release() to guarantee proper ordering between stores > initializing object attributes and the store initializing the refcount. > refcount_set_release() should be done after all other object attributes > are initialized. Once refcount_set_release() is called, the object should > be considered visible to other tasks even if it was not yet added into an > object collection normally used to discover it. This is because other > tasks might have discovered the object previously occupying the same > memory and after memory reuse they can succeed in taking refcount for the > new object and start using it. > > Object reuse example to consider: > > consumer: > obj = lookup(collection, key); > if (!refcount_inc_not_zero_acquire(&obj->ref)) > return; > if (READ_ONCE(obj->key) != key) { /* identity check */ > put_ref(obj); > return; > } > use(obj->value); > > producer: > remove(collection, obj->key); > if (!refcount_dec_and_test(&obj->ref)) > return; > obj->key = KEY_INVALID; > free(obj); > obj = malloc(); /* obj is reused */ > obj->key = new_key; > obj->value = new_value; > refcount_set_release(obj->ref, 1); > add(collection, new_key, obj); > > refcount_{add|inc}_not_zero_acquire() is required to prevent the following > reordering when refcount_inc_not_zero() is used instead: > > consumer: > obj = lookup(collection, key); > if (READ_ONCE(obj->key) != key) { /* reordered identity check */ > put_ref(obj); > return; > } > producer: > remove(collection, obj->key); > if (!refcount_dec_and_test(&obj->ref)) > return; > obj->key = KEY_INVALID; > free(obj); > obj = malloc(); /* obj is reused */ > obj->key = new_key; > obj->value = new_value; > refcount_set_release(obj->ref, 1); > add(collection, new_key, obj); > > if (!refcount_inc_not_zero(&obj->ref)) > return; > use(obj->value); /* USING WRONG OBJECT */ > > refcount_set_release() is required to prevent the following reordering > when refcount_set() is used instead: > > consumer: > obj = lookup(collection, key); > > producer: > remove(collection, obj->key); > if (!refcount_dec_and_test(&obj->ref)) > return; > obj->key = KEY_INVALID; > free(obj); > obj = malloc(); /* obj is reused */ > obj->key = new_key; /* new_key == old_key */ > refcount_set(obj->ref, 1); > > if (!refcount_inc_not_zero_acquire(&obj->ref)) > return; > if (READ_ONCE(obj->key) != key) { /* pass since new_key == old_key */ > put_ref(obj); > return; > } > use(obj->value); /* USING STALE obj->value */ > > obj->value = new_value; /* reordered store */ > add(collection, key, obj); > > Signed-off-by: Suren Baghdasaryan > Cc: Peter Zijlstra > Cc: Will Deacon > Cc: Paul E. McKenney Acked-by: Vlastimil Babka #slab