Date: Tue, 16 Oct 2018 02:30:13 +0000 (UTC) From: "Jonathan T. Looney" <jtl@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r339375 - head/sys/contrib/ck/src Message-ID: <201810160230.w9G2UD4P064685@repo.freebsd.org>
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Author: jtl Date: Tue Oct 16 02:30:13 2018 New Revision: 339375 URL: https://svnweb.freebsd.org/changeset/base/339375 Log: Import CK as of commit 5221ae2f3722a78c7fc41e47069ad94983d3bccb. This fixes two problems, one where epoch calls could occur before all the readers had exited the epoch section, and one where the epoch calls could be unnecessarily delayed. Approved by: re (glebius) Modified: head/sys/contrib/ck/src/ck_epoch.c Directory Properties: head/sys/contrib/ck/ (props changed) Modified: head/sys/contrib/ck/src/ck_epoch.c ============================================================================== --- head/sys/contrib/ck/src/ck_epoch.c Tue Oct 16 00:50:00 2018 (r339374) +++ head/sys/contrib/ck/src/ck_epoch.c Tue Oct 16 02:30:13 2018 (r339375) @@ -127,6 +127,14 @@ */ #define CK_EPOCH_GRACE 3U +/* + * CK_EPOCH_LENGTH must be a power-of-2 (because (CK_EPOCH_LENGTH - 1) is used + * as a mask, and it must be at least 3 (see comments above). + */ +#if (CK_EPOCH_LENGTH < 3 || (CK_EPOCH_LENGTH & (CK_EPOCH_LENGTH - 1)) != 0) +#error "CK_EPOCH_LENGTH must be a power of 2 and >= 3" +#endif + enum { CK_EPOCH_STATE_USED = 0, CK_EPOCH_STATE_FREE = 1 @@ -348,7 +356,7 @@ ck_epoch_scan(struct ck_epoch *global, return NULL; } -static void +static unsigned int ck_epoch_dispatch(struct ck_epoch_record *record, unsigned int e, ck_stack_t *deferred) { unsigned int epoch = e & (CK_EPOCH_LENGTH - 1); @@ -366,6 +374,7 @@ ck_epoch_dispatch(struct ck_epoch_record *record, unsi ck_stack_push_spnc(deferred, &entry->stack_entry); else entry->function(entry); + i++; } @@ -381,7 +390,7 @@ ck_epoch_dispatch(struct ck_epoch_record *record, unsi ck_pr_sub_uint(&record->n_pending, i); } - return; + return i; } /* @@ -560,16 +569,28 @@ ck_epoch_poll_deferred(struct ck_epoch_record *record, unsigned int epoch; struct ck_epoch_record *cr = NULL; struct ck_epoch *global = record->global; + unsigned int n_dispatch; epoch = ck_pr_load_uint(&global->epoch); /* Serialize epoch snapshots with respect to global epoch. */ ck_pr_fence_memory(); + + /* + * At this point, epoch is the current global epoch value. + * There may or may not be active threads which observed epoch - 1. + * (ck_epoch_scan() will tell us that). However, there should be + * no active threads which observed epoch - 2. + * + * Note that checking epoch - 2 is necessary, as race conditions can + * allow another thread to increment the global epoch before this + * thread runs. + */ + n_dispatch = ck_epoch_dispatch(record, epoch - 2, deferred); + cr = ck_epoch_scan(global, cr, epoch, &active); - if (cr != NULL) { - record->epoch = epoch; - return false; - } + if (cr != NULL) + return (n_dispatch > 0); /* We are at a grace period if all threads are inactive. */ if (active == false) { @@ -580,10 +601,17 @@ ck_epoch_poll_deferred(struct ck_epoch_record *record, return true; } - /* If an active thread exists, rely on epoch observation. */ + /* + * If an active thread exists, rely on epoch observation. + * + * All the active threads entered the epoch section during + * the current epoch. Therefore, we can now run the handlers + * for the immediately preceding epoch and attempt to + * advance the epoch if it hasn't been already. + */ (void)ck_pr_cas_uint(&global->epoch, epoch, epoch + 1); - ck_epoch_dispatch(record, epoch + 1, deferred); + ck_epoch_dispatch(record, epoch - 1, deferred); return true; }
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