mutexes-locks-annotations.txt

LOCKS, MUTEXES, ANNOTATIONS

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sipa:     Annotations vs assertions: they're somewhat orthogonal

          Annotations:

          [+] Compile-time check. Guarantees absence of issues in every possible code path.
          [-] Only works in Clang.
          [-] Can't be used in some more advanced locking scenarios.

          Assertions:

          [+] Works in GCC and Clang
          [+] Isn't restricted to analyzable cases
          [-] Is only a runtime check; it needs test cases that actually exercise the bug
          [-] Needs building with -DDEBUG_LOCKORDER

          Perhaps at some point it's time to evaluate if we can pick just one
          over the other (which I guess would be the annotations approach), but
          they're not exactly identical.
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Discussion in https://github.com/bitcoin/bitcoin/pull/16127

sipa:     Our "default" mutexes are recursive mutexes (which you can enter
          multiple times from the same thread). They're easier to work with as
          you don't need to worry about whether to call a lock-grabbing function
          from either inside or outside the lock. However, recursive mutexes
          can't be used for condition variable waiting, so for those, we use
          std::mutex (which is not recursive) directly.

          Longer term we should try to get rid of recursive mutexes entirely;
          code is much cleaner without them.


ajtowns   > MarcoFalke: How is this different from using Mutex m; and LOCK(m); ?

          If you do Mutex cs; instead of std::mutex cs; then you can do
          LOCK(cs); or WAIT_LOCK(cs, g); instead of MutexGuard g(cs);. In that
          case the type of g changes from a trivial subclass of std::lock_guard
          to UniqueLock which is a subclass of std::unique_lock that supports
          DEBUG_LOCKCONTENTION in which case it uses LogPrintf to report
          potential deadlocks or lock contention. Even without the extra
          features we have, std::unique_lock is potentially higher overhead than
          std::lock_guard, but std::lock_guard isn't unlockable except on
          destruction, so unique_lock is needed for use with condition
          variables.

          - We can't use Mutex for locking for the logging API, because that is
            a circular dependency.

          - We use std::mutex in code that implements our specialised locking
            behaviour got Mutex etc.

          - We don't use Mutex in support/lockedpool.h, probably because at
            present it is independent of bitcoin interfaces in general (this PR
            adds a dependency on threadsafety.h, so maybe just using Mutex would
            make sense at that point).

          - We use std::mutex instead of Mutex for protecting dir_locks in
            util/system.h, not sure if there's a circular dependency there to
            justify it though?

          - We also use std::mutex in some unit tests, which I don't think
            matters much.

          There's some cases where we currently use std::lock_guard<std::mutex>
          g(cs) where cs is a Mutex, rather than calling LOCK(cs); or
          WAIT_LOCK(cs, g); That saves a bit of overhead, while losing our
          contention/deadlock debugging info, which I'm not sure makes much
          sense. I've updated this PR to change those instances to just use
          LOCK(cs) instead of lock_guard.

          I think that means it makes sense to:

          - Use Mutex and LOCK etc as the "normal" locking mechanism.

          - Use RecursiveMutex (aka CCriticalSection) if recursive locks are
            needed, or it's old code that we haven't worked through yet.

          - Use std::mutex and { MutexGuard guard(mutex); ... } for code that's
            used to implement Mutex

          I've bumped this PR to replace a couple of the lock_guard instances
          that referred to our Mutex/CCriticalSection classes with LOCK instead
          of MutexGuard.

          > ryanofsky: I don't think it's true that we can't allow a circular
          dependency, but maybe you can explain more.

          Maybe; LOCK(m); becomes a call to UniqueLock constructor, which calls
          Enter() (or TryEnter()), which calls EnterCritical(), which calls
          push_lock() which may call potential_deadlock_detected() which then
          calls LogPrintf, which calls LogPrintStr which, when logging to a
          file, will then lock m_file_mutex. But I don't think that actually
          causes a problem, because I don't think any lock is ever taken while
          the m_file_mutex is held, so that shouldn't trigger
          potential_deadlock_detected.

          There's also the case if you compile with -DDEBUG_LOCKCONTENTION. If
          two processes try to log simultaneously, the first one will lock
          m_file_mutex successfully, while the second one will fail at the
          try_lock phase of UniqueLock::Enter, which will then call
          PrintLockContention and hence LogPrintf, and recursively fail for the
          same reason until m_file_mutex is released, at which point things
          should clean up.

          > ryanofsky: I think ideally, we would just use Mutex and RecursiveMutex
          everywhere, since they fully support LOCK macros, lock assertions,
          lock annotations, deadlock detection, and so on. What exactly is the
          need for std::mutex and MutexGuard?

          If we can switch all the uses of std::mutex to Mutex, I don't think
          there's a need for MutexGuard.
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jnewbery: We now have a handy WITH_LOCK macro in src/sync.h#L209

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