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roots: quick fix for task scheduling bug leading to loss of crawl_master

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The crawl_master task had a simple atomic variable that was supposed
to prevent duplicate crawl_master tasks from ending up in the queue;
however, this had a race condition that could lead to m_task_running
being set with no crawl_master task running to clear it.  This would in
turn prevent crawl_thread from scheduling any further crawl_master tasks,
and bees would eventually stop doing any more work.

A proper fix is to modify the Task class and its friends such that
Task::run() guarantees that 1) at most one instance of a Task is ever
scheduled or running at any time, and 2) if a Task is scheduled while
an instance of the Task is running, the scheduling is deferred until
after the current instance completes.  This is part of a fairly large
planned change set, but it's not ready to push now.

So instead, unconditionally push a new crawl_master Task into the queue
on every poll, then silently and quickly exit if the queue is too full
or the supply of new extents is empty.  Drop the scheduling-related
members of BeesRoots as they will not be needed when the proper fix lands.

Fixes: 4f0bc78a "crawl: don't block a Task waiting for new transids"
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2018-11-24 00:35:16 -05:00
parent f051d96d51
commit f4464c6896
2 changed files with 11 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
src/bees-roots.cc
View File

@ -392,15 +392,22 @@ BeesRoots::crawl_thread()
m_crawl_task = Task("crawl_master", [shared_this]() {
auto tqs = TaskMaster::get_queue_count();
BEESNOTE("queueing extents to scan, " << tqs << " of " << BEES_MAX_QUEUE_SIZE);
#if 0
bool run_again = true;
while (tqs < BEES_MAX_QUEUE_SIZE && run_again) {
run_again = shared_this->crawl_roots();
tqs = TaskMaster::get_queue_count();
}
#else
bool run_again = false;
while (tqs < BEES_MAX_QUEUE_SIZE) {
run_again = shared_this->crawl_roots();
tqs = TaskMaster::get_queue_count();
if (!run_again) break;
}
#endif
if (run_again) {
shared_this->m_crawl_task.run();
} else {
shared_this->m_task_running = false;
}
});
@ -429,12 +436,7 @@ BeesRoots::crawl_thread()
last_count = new_count;
// If no crawl task is running, start a new one
bool already_running = m_task_running.exchange(true);
if (!already_running) {
auto resumed_after_time = m_crawl_timer.lap();
BEESLOGINFO("Crawl master resumed after " << resumed_after_time << "s at transid " << new_count);
m_crawl_task.run();
}
auto poll_time = m_transid_re.seconds_for(m_transid_factor);
BEESLOGDEBUG("Polling " << poll_time << "s for next " << m_transid_factor << " transid " << m_transid_re);
@ -554,8 +556,7 @@ BeesRoots::BeesRoots(shared_ptr<BeesContext> ctx) :
m_ctx(ctx),
m_crawl_state_file(ctx->home_fd(), crawl_state_filename()),
m_crawl_thread("crawl_transid"),
m_writeback_thread("crawl_writeback"),
m_task_running(false)
m_writeback_thread("crawl_writeback")
{
m_root_ro_cache.func([&](uint64_t root) -> bool {

1
src/bees.h
View File

@ -525,7 +525,6 @@ class BeesRoots : public enable_shared_from_this<BeesRoots> {
BeesThread m_writeback_thread;
RateEstimator m_transid_re;
size_t m_transid_factor = BEES_TRANSID_FACTOR;
atomic<bool> m_task_running;
Task m_crawl_task;
bool m_workaround_btrfs_send = false;
LRUCache<bool, uint64_t> m_root_ro_cache;