Author: mmacy
Date: Thu Aug 9 05:18:27 2018
New Revision: 337525
URL: https://svnweb.freebsd.org/changeset/base/337525
Log:
epoch_block_wait: don't check TD_RUNNING
struct epoch_thread is not type safe (stack allocated) and thus cannot be
dereferenced from another CPU
Reported by: novel@
Modified:
head/sys/kern/subr_epoch.c
Modified: head/sys/kern/subr_epoch.c
==============================================================================
--- head/sys/kern/subr_epoch.c Thu Aug 9 03:45:47 2018 (r337524)
+++ head/sys/kern/subr_epoch.c Thu Aug 9 05:18:27 2018 (r337525)
@@ -56,7 +56,7 @@ __FBSDID("$FreeBSD$");
static MALLOC_DEFINE(M_EPOCH, "epoch", "epoch based reclamation");
/* arbitrary --- needs benchmarking */
-#define MAX_ADAPTIVE_SPIN 1000
+#define MAX_ADAPTIVE_SPIN 100
#define MAX_EPOCHS 64
CTASSERT(sizeof(ck_epoch_entry_t) == sizeof(struct epoch_context));
@@ -240,24 +240,38 @@ epoch_block_handler_preempt(struct ck_epoch *global __
locksheld = td->td_locks;
spincount = 0;
counter_u64_add(block_count, 1);
+ /*
+ * We lost a race and there's no longer any threads
+ * on the CPU in an epoch section.
+ */
+ if (TAILQ_EMPTY(&record->er_tdlist))
+ return;
+
if (record->er_cpuid != curcpu) {
/*
* If the head of the list is running, we can wait for it
* to remove itself from the list and thus save us the
* overhead of a migration
*/
- if ((tdwait = TAILQ_FIRST(&record->er_tdlist)) != NULL &&
- TD_IS_RUNNING(tdwait->et_td)) {
- gen = record->er_gen;
- thread_unlock(td);
- do {
- cpu_spinwait();
- } while (tdwait == TAILQ_FIRST(&record->er_tdlist) &&
- gen == record->er_gen &&
TD_IS_RUNNING(tdwait->et_td) &&
- spincount++ < MAX_ADAPTIVE_SPIN);
- thread_lock(td);
+ gen = record->er_gen;
+ thread_unlock(td);
+ /*
+ * We can't actually check if the waiting thread is running
+ * so we simply poll for it to exit before giving up and
+ * migrating.
+ */
+ do {
+ cpu_spinwait();
+ } while (!TAILQ_EMPTY(&record->er_tdlist) &&
+ gen == record->er_gen &&
+ spincount++ < MAX_ADAPTIVE_SPIN);
+ thread_lock(td);
+ /*
+ * If the generation has changed we can poll again
+ * otherwise we need to migrate.
+ */
+ if (gen != record->er_gen)
return;
- }
/*
* Being on the same CPU as that of the record on which
* we need to wait allows us access to the thread
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