-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 Hi everyone,
I'm David from the LTTng project (http://lttng.org). I'm sharing here an issue we ran into. The solution could benefit the libevent project. A couple days ago we've hit a clock race using pthread timedwait conditions (although documented in the man page). We've found a solution and I wanted to share it with the libevent community since I've checked and it seems that it can happen in libevent. Here goes. The pthread_cond_timedwait() needs an absolute timeout value, generally taken from the realtime clock of the system (using gettimeofday or clock_gettime(CLOCK_REALTIME,...)). One point to mention is that this clock can go backward or forward at any time with for instance a NTP adjustment (no new knowledge here). So, the race occurred between the clock time acquisition and the pthread_cond_timedwait() call where it immediately returned with a ETIMEOUT (normal behavior) but breaking a good part of the boot process of our daemon. By the time the pthread cond call was reschedule, the system had a NTP adjustment thus making the timeout period invalid. We've hit that race at a massive cluster reboot where only a couple of nodes were affected by it since it's highly unlikely to encounter but still possible. After some research, we discovered a fantastic call, pthread_condattr_setclock() which can be used to set the condition to use the CLOCK_MONOTONIC hence having the guarantee to never jump in time between calls. Now, I know this is not new stuff but the fact that we've hit that recently on large clusters of machines makes me think that it could be very interesting that libevent cond wait call could avoid this race. (evthread_posix_cond_wait). Thoughts? Thanks everyone! Cheers David -----BEGIN PGP SIGNATURE----- iQEcBAEBCgAGBQJQWNYtAAoJEELoaioR9I027a0H/2wPAKVRH63zsIX2QVpwL76p VfJGHk8b2S9i1av55JTyt8YmN+iXE1U7Qf7bNQA0VUy59Ykd5V+cqSLDbRCnfPog fSeZyMFacWKHQ36JsbSb77xKYkMAC0d9n+Na+JRXjc4jdv5eTiI9vlCQDhyXFR9t i4XHqDB97PJh/2xcecYlgtbOM5QeLcglDVQpcRIwhHzC9yx0eLKiPgU6+uMMequO JK6e+N8pElwf8bywE/NnmLM2/NpWqwr7i3IbC+Zi8vgcI2yG+7rp/En+/B4SPA0m qVGTV4NHAWTY4cmMxvJY9PXPc5l3Sp/sjziBZTlCgsMpT5Cnd/ygOaZGHnjq3T8= =zUj4 -----END PGP SIGNATURE----- *********************************************************************** To unsubscribe, send an e-mail to majord...@freehaven.net with unsubscribe libevent-users in the body.
