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some cases we want to improve:
1) if a thread signal got a signal while in cancellation point,
it is possible the TDP_WAKEUP may be eaten by signal handler
if the handler called some interruptibly system calls.
2) In signal handler, we want to disable cancellation.
3) When thread holding some low level locks, it is better to
disable signal, those code need not to worry reentrancy,
sigprocmask system call is avoided because it is a bit expensive.
The signal handler wrapper works in this way:
1) libthr installs its signal handler if user code invokes sigaction
to install its handler, the user handler is recorded in internal
array.
2) when a signal is delivered, libthr's signal handler is invoke,
libthr checks if thread holds some low level lock or is in critical
region, if it is true, the signal is buffered, and all signals are
masked, once the thread leaves critical region, correct signal
mask is restored and buffered signal is processed.
3) before user signal handler is invoked, cancellation is temporarily
disabled, after user signal handler is returned, cancellation state
is restored, and pending cancellation is rescheduled.
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which does not know what is the state of interrupted system call, for
example, open() system call opened a file and the thread is still cancelled,
result is descriptor leak, there are other problems which can cause resource
leak or undeterminable side effect when a thread is cancelled. However, this
is no longer true in new implementation.
In defering mode, a thread is canceled if cancellation request is pending and
later the thread enters a cancellation point, otherwise, a later
pthread_cancel() just causes SIGCANCEL to be sent to the target thread, and
causes target thread to abort system call, userland code in libthr then checks
cancellation state, and cancels the thread if needed. For example, the
cancellation point open(), the thread may be canceled at start,
but later, if it opened a file descriptor, it is not canceled, this avoids
file handle leak. Another example is read(), a thread may be canceled at start
of the function, but later, if it read some bytes from a socket, the thread
is not canceled, the caller then can decide if it should still enable cancelling
or disable it and continue reading data until it thinks it has read all
bytes of a packet, and keeps a protocol stream in health state, if user ignores
partly reading of a packet without disabling cancellation, then second iteration
of read loop cause the thread to be cancelled.
An exception is that the close() cancellation point always closes a file handle
despite whether the thread is cancelled or not.
The old mechanism is still kept, for a functions which is not so easily to
fix a cancellation problem, the rough mechanism is used.
Reviewed by: kib@
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is also returned by pthread_detach() if a thread was already
detached, the error code was already documented:
> [EINVAL] The implementation has detected that the value speci-
> fied by thread does not refer to a joinable thread.
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reduce overheads of cancellation points.
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Bug reported by: jasone at connonware when using ports lang/onyx
MFC after: 3 days
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address, let kernel wake it up.
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except the function will return ETIMEDOUT if target thread does not exit
before specified absolute time passes.
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1. fast simple type mutex.
2. __thread tls works.
3. asynchronous cancellation works ( using signal ).
4. thread synchronization is fully based on umtx, mainly, condition
variable and other synchronization objects were rewritten by using
umtx directly. those objects can be shared between processes via
shared memory, it has to change ABI which does not happen yet.
5. default stack size is increased to 1M on 32 bits platform, 2M for
64 bits platform.
As the result, some mysql super-smack benchmarks show performance is
improved massivly.
Okayed by: jeff, mtm, rwatson, scottl
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no userland locks are heald, the dead thread lock can no longer protect
access to it. Therefore, instead of using an if (!dead)...else clause
after walking the active threads list test the thread pointer before
deciding not to walk the dead threads list. If the thread pointer is null
it means it was not found in the active threads list and the dead threads
list should be checked.
2. Do not free the stack of a thread that is not marked dead. This is the
2nd and final part of eliminating the race to free a thread's stack.
MFC after: 3 days
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followed are: Only 3 functions (pthread_cancel, pthread_setcancelstate,
pthread_setcanceltype) are required to be async-signal-safe by POSIX. None of
the rest of the pthread api is required to be async-signal-safe. This means
that only the three mentioned functions are safe to use from inside
signal handlers.
However, there are certain system/libc calls that are
cancellation points that a caller may call from within a signal handler,
and since they are cancellation points calls have to be made into libthr
to test for cancellation and exit the thread if necessary. So, the
cancellation test and thread exit code paths must be async-signal-safe
as well. A summary of the changes follows:
o Almost all of the code paths that masked signals, as well as locking the
pthread structure now lock only the pthread structure.
o Signals are masked (and left that way) as soon as a thread enters
pthread_exit().
o The active and dead threads locks now explicitly require that signals
are masked.
o Access to the isdead field of the pthread structure is protected by both
the active and dead list locks for writing. Either one is sufficient for
reading.
o The thread state and type fields have been combined into one three-state
switch to make it easier to read without requiring a lock. It doesn't need
a lock for writing (and therefore for reading either) because only the
current thread can write to it and it is an integer value.
o The thread state field of the pthread structure has been eliminated. It
was an unnecessary field that mostly duplicated the flags field, but
required additional locking that would make a lot more code paths require
signal masking. Any truly unique values (such as PS_DEAD) have been
reborn as separate members of the pthread structure.
o Since the mutex and condvar pthread functions are not async-signal-safe
there is no need to muck about with the wait queues when handling
a signal ...
o ... which also removes the need for wrapping signal handlers and sigaction(2).
o The condvar and mutex async-cancellation code had to be revised as a result
of some of these changes, which resulted in semi-unrelated changes which
would have been difficult to work on as a separate commit, so they are
included as well.
The only part of the changes I am worried about is related to locking for
the pthread joining fields. But, I will take a closer look at them once this
mega-patch is committed.
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responsibility.
o If a thread is not joinable, the correct return value is EINVAL.
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in-line. If the exiting thread cannot release a resource, then
the next thread to exit will release it.
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It is a more acurate description of the locks they
operate on.
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libthr.
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The dead list thread is sufficient for synchronization.
Retire the arch_id (ldt array slot) in the gc thread instead of the
doing it in the thread itself.
Approved by: re/jhb
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Approved by: re/rwatson
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thread is not dead, the join loop is guaranteed to execute at least
once, so there is no need to pick up the thread list lock after
we return from suspenstion only to release it after the loop.
Approved by: re/blanket libthr
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list is protected by a spinlock_t, but the dead list uses a pthread_mutex
because it is necessary to synchronize other threads with the garbage
collector thread. Lock/Unlock macros are used so it's easier to make
changes to the locks in the future.
The 'dead thread list' lock is intended to replace the gc mutex.
This doesn't have any practical ramifications. It simply makes it
clearer what the purpose of the lock is. The gc will use this lock,
instead of the gc mutex, to synchronize access to the dead list with
other threads.
Modify _pthread_exit() to use these two new locks instead of GIANT_LOCK,
and also to properly lock and protect thread state changes,
especially with respect to a joining thread.
The gc thread was also re-arranged to be more organized and less nested.
_pthread_join() was also modified to use the thread list locks. However,
locking and unlocking here needs special care because a thread could find
itself in a position where it's joining an exiting thread that is
waiting on the dead list lock, which this thread (joiner) holds. If the
joiner doesn't take care to lock *and* unlock in the same order they
(the joiner and the joinee) could deadlock against each other.
Approved by: re/blanket libthr
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Approved by: markm/mentor (implicit), re/blanket libthr
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_get_curthread(). This is similar to the kernel's curthread. Doing
this saves stack overhead and is more convenient to the programmer.
- Pass the pointer to the newly created thread to _thread_init().
- Remove _get_curthread_slow().
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this manually. This will facilitate the unrolling of giant.
- Don't allow giant to recurse anymore. This should never happen.
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adaptation of libc_r for the thr system call interface. This is beta
quality code.
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