mirror of
https://github.com/matrix-construct/construct
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96 lines
4.5 KiB
C++
96 lines
4.5 KiB
C++
// Matrix Construct
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//
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// Copyright (C) Matrix Construct Developers, Authors & Contributors
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// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
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//
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// Permission to use, copy, modify, and/or distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice is present in all copies. The
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// full license for this software is available in the LICENSE file.
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#if __has_include("db/write_thread.h")
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#define ROCKSDB_PLATFORM_POSIX
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#include "db/write_thread.h"
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#endif
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#ifndef STORAGE_ROCKSDB_INCLUDE_DB_H_
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#warning "The RocksDB source code is not available. Cannot interpose bugfixes for db/write_thread.h."
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#endif
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///////////////////////////////////////////////////////////////////////////////
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//
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// This unit exists to mitigate an instance of a bug in RocksDB documented by
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// https://github.com/facebook/rocksdb/issues/4654. In summary, some RocksDB
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// code makes direct use of std::mutex and std::condition_variable unlike the
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// rest of RocksDB code which uses the rocksdb::port and rocksdb::Env wrapper
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// interfaces. We have adapted the latter to work with ircd::ctx userspace
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// threading (see: db_port.cc and db_env.cc), but the former is a direct
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// interface to kernel pthreads which are incompatible in this context.
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//
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// Our mitigation is made possible by dynamic linking. It is a legitimate use
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// of runtime interposition as stated in official documentation for this exact
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// purpose: overriding buggy functions in library dependencies.
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//
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// This unit overrides a class member function in rocksdb::WriteThread which
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// originally made use of pthread primitives to handle two threads contending
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// for write access in RocksDB's single-writer design. This function is entered
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// by additional threads after a first thread is an established "write leader."
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// These additional threads wait until a state bitmask satisfies them so they
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// can continue. This waiting is accomplished with an std::condition_variable
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// which tells the kernel to stop the thread until satisfied. Since we are not
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// using kernel-driven threads, this is a deadlock.
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//
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// Our solution is not ideal, but operates effectively and is rarely invoked
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// given our prior mitigations to eliminate write contention inside RocksDB.
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// This code may be hit during simultaneous flush and compaction jobs, which is
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// a rare event with our configuration. When hit, our yield loop tends to have
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// one iteration. Our solution using a coarse yield, though, is not ideal: the
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// waiting ircd::ctx is not actually being notified of a state mask change
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// because the write-leader is notifying an std::condition_variable which leads
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// to nowhere. Our coarse yield simply puts the waiting ircd::ctx at the back
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// of the ircd::ios queue (similar to a sched_yield(2)) and hopes that the
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// write-leader has finished its task and updated the state mask before the
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// waiter reaches the front of the queue to check. In the worst-case, the
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// waiter will requeue itself to check the state mask too often. We can make
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// further improvements here by also overriding the notification mechanism
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// as well, but we do not feel it's required at this time, as noted.
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//
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// Unfortunately, we are overriding a C++ class member function to achieve this.
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// Class member functions require the correct class definition. This means we
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// require a copy of the RocksDB source code to be accessible at build-time. If
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// no copy of the RocksDB source is found when this unit is built, it is not an
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// error, but the mitigation will not be made and the result will be vulnerable
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// to this deadlock.
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#ifdef STORAGE_ROCKSDB_INCLUDE_DB_H_
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uint8_t
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rocksdb::WriteThread::BlockingAwaitState(Writer *const w,
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uint8_t goal_mask)
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{
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// Create the class member mutex and cv where it's expected by
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// rocksdb callers
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w->CreateMutex();
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auto state(w->state.load(std::memory_order_acquire));
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assert(state != STATE_LOCKED_WAITING);
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if((state & goal_mask) == 0 && w->state.compare_exchange_strong(state, STATE_LOCKED_WAITING))
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{
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size_t yields(0);
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while((state = w->state.load(std::memory_order_relaxed)) == STATE_LOCKED_WAITING)
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{
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ircd::ctx::yield();
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++yields;
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}
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// Since we're using a coarse ctx::yield() it's theoretically possible
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// that our loop can spin out of control. That is highly unlikely,
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// and there is usually not even more than one iteration. Nevertheless
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// we assert to be sure this is working within reason.
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assert(yields < 32);
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}
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assert((state & goal_mask) != 0);
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return state;
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}
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#endif STORAGE_ROCKSDB_INCLUDE_DB_H_
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