commit 0ff395c154ad827c0c30eefc9371ba7f7c171027 Author: Willy Tarreau Date: Tue Jul 30 11:59:34 2019 +0200 BUG/MAJOR: queue/threads: avoid an AB/BA locking issue in process_srv_queue() A problem involving server slowstart was reported by @max2k1 in issue #197. The problem is that pendconn_grab_from_px() takes the proxy lock while already under the server's lock while process_srv_queue() first takes the proxy's lock then the server's lock. While the latter seems more natural, it is fundamentally incompatible with mayn other operations performed on servers, namely state change propagation, where the proxy is only known after the server and cannot be locked around the servers. Howwever reversing the lock in process_srv_queue() is trivial and only the few functions related to dynamic cookies need to be adjusted for this so that the proxy's lock is taken for each server operation. This is possible because the proxy's server list is built once at boot time and remains stable. So this is what this patch does. The comments in the proxy and server structs were updated to mention this rule that the server's lock may not be taken under the proxy's lock but may enclose it. Another approach could consist in using a second lock for the proxy's queue which would be different from the regular proxy's lock, but given that the operations above are rare and operate on small servers list, there is no reason for overdesigning a solution. This fix was successfully tested with 10000 servers in a backend where adjusting the dyncookies in loops over the CLI didn't have a measurable impact on the traffic. The only workaround without the fix is to disable any occurrence of "slowstart" on server lines, or to disable threads using "nbthread 1". This must be backported as far as 1.8. (cherry picked from commit 5e83d996cf965ee5ac625f702a446f4d8c80a220) Signed-off-by: Willy Tarreau diff --git a/include/types/proxy.h b/include/types/proxy.h index ca24dbfe..2518f88d 100644 --- a/include/types/proxy.h +++ b/include/types/proxy.h @@ -487,7 +487,7 @@ struct proxy { * name is used */ struct list filter_configs; /* list of the filters that are declared on this proxy */ - __decl_hathreads(HA_SPINLOCK_T lock); + __decl_hathreads(HA_SPINLOCK_T lock); /* may be taken under the server's lock */ }; struct switching_rule { diff --git a/include/types/server.h b/include/types/server.h index 4a077268..e0534162 100644 --- a/include/types/server.h +++ b/include/types/server.h @@ -319,7 +319,7 @@ struct server { } ssl_ctx; #endif struct dns_srvrq *srvrq; /* Pointer representing the DNS SRV requeest, if any */ - __decl_hathreads(HA_SPINLOCK_T lock); + __decl_hathreads(HA_SPINLOCK_T lock); /* may enclose the proxy's lock, must not be taken under */ struct { const char *file; /* file where the section appears */ struct eb32_node id; /* place in the tree of used IDs */ diff --git a/src/proxy.c b/src/proxy.c index ae761ead..a537e0b1 100644 --- a/src/proxy.c +++ b/src/proxy.c @@ -1940,9 +1940,12 @@ static int cli_parse_enable_dyncookie_backend(char **args, char *payload, struct if (!px) return 1; + /* Note: this lock is to make sure this doesn't change while another + * thread is in srv_set_dyncookie(). + */ HA_SPIN_LOCK(PROXY_LOCK, &px->lock); - px->ck_opts |= PR_CK_DYNAMIC; + HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); for (s = px->srv; s != NULL; s = s->next) { HA_SPIN_LOCK(SERVER_LOCK, &s->lock); @@ -1950,8 +1953,6 @@ static int cli_parse_enable_dyncookie_backend(char **args, char *payload, struct HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock); } - HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); - return 1; } @@ -1971,9 +1972,12 @@ static int cli_parse_disable_dyncookie_backend(char **args, char *payload, struc if (!px) return 1; + /* Note: this lock is to make sure this doesn't change while another + * thread is in srv_set_dyncookie(). + */ HA_SPIN_LOCK(PROXY_LOCK, &px->lock); - px->ck_opts &= ~PR_CK_DYNAMIC; + HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); for (s = px->srv; s != NULL; s = s->next) { HA_SPIN_LOCK(SERVER_LOCK, &s->lock); @@ -1984,8 +1988,6 @@ static int cli_parse_disable_dyncookie_backend(char **args, char *payload, struc HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock); } - HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); - return 1; } @@ -2021,10 +2023,13 @@ static int cli_parse_set_dyncookie_key_backend(char **args, char *payload, struc return 1; } + /* Note: this lock is to make sure this doesn't change while another + * thread is in srv_set_dyncookie(). + */ HA_SPIN_LOCK(PROXY_LOCK, &px->lock); - free(px->dyncookie_key); px->dyncookie_key = newkey; + HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); for (s = px->srv; s != NULL; s = s->next) { HA_SPIN_LOCK(SERVER_LOCK, &s->lock); @@ -2032,8 +2037,6 @@ static int cli_parse_set_dyncookie_key_backend(char **args, char *payload, struc HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock); } - HA_SPIN_UNLOCK(PROXY_LOCK, &px->lock); - return 1; } diff --git a/src/queue.c b/src/queue.c index f4a94530..6aa54170 100644 --- a/src/queue.c +++ b/src/queue.c @@ -312,16 +312,16 @@ void process_srv_queue(struct server *s) struct proxy *p = s->proxy; int maxconn; - HA_SPIN_LOCK(PROXY_LOCK, &p->lock); HA_SPIN_LOCK(SERVER_LOCK, &s->lock); + HA_SPIN_LOCK(PROXY_LOCK, &p->lock); maxconn = srv_dynamic_maxconn(s); while (s->served < maxconn) { int ret = pendconn_process_next_strm(s, p); if (!ret) break; } - HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock); HA_SPIN_UNLOCK(PROXY_LOCK, &p->lock); + HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock); } /* Adds the stream to the pending connection queue of server ->srv @@ -424,7 +424,8 @@ int pendconn_redistribute(struct server *s) /* Check for pending connections at the backend, and assign some of them to * the server coming up. The server's weight is checked before being assigned * connections it may not be able to handle. The total number of transferred - * connections is returned. + * connections is returned. It must be called with the server lock held, and + * will take the proxy's lock. */ int pendconn_grab_from_px(struct server *s) { diff --git a/src/server.c b/src/server.c index a96f1ef6..236d6bae 100644 --- a/src/server.c +++ b/src/server.c @@ -125,7 +125,7 @@ static inline void srv_check_for_dup_dyncookie(struct server *s) } /* - * Must be called with the server lock held. + * Must be called with the server lock held, and will grab the proxy lock. */ void srv_set_dyncookie(struct server *s) { @@ -137,15 +137,17 @@ void srv_set_dyncookie(struct server *s) int addr_len; int port; + HA_SPIN_LOCK(PROXY_LOCK, &p->lock); + if ((s->flags & SRV_F_COOKIESET) || !(s->proxy->ck_opts & PR_CK_DYNAMIC) || s->proxy->dyncookie_key == NULL) - return; + goto out; key_len = strlen(p->dyncookie_key); if (s->addr.ss_family != AF_INET && s->addr.ss_family != AF_INET6) - return; + goto out; /* * Buffer to calculate the cookie value. * The buffer contains the secret key + the server IP address @@ -174,7 +176,7 @@ void srv_set_dyncookie(struct server *s) hash_value = XXH64(tmpbuf, buffer_len, 0); memprintf(&s->cookie, "%016llx", hash_value); if (!s->cookie) - return; + goto out; s->cklen = 16; /* Don't bother checking if the dyncookie is duplicated if @@ -183,6 +185,8 @@ void srv_set_dyncookie(struct server *s) */ if (!(s->next_admin & SRV_ADMF_FMAINT)) srv_check_for_dup_dyncookie(s); + out: + HA_SPIN_UNLOCK(PROXY_LOCK, &p->lock); } /*