this is a "nonstandard" function that was "rejected" by POSIX, but
nonetheless had its behavior documented in the POSIX rationale for
fork. it's present on solaris and possibly some other systems, and
duplicates the whole calling process, not just a single thread. glibc
does not have this function. it should not be used in programs
intending to be portable, but may be useful for testing,
checkpointing, etc. and it's an interesting (and quite small) example
of the usefulness of the __synccall framework originally written to
work around deficiencies in linux's setuid syscall.
fix up clone signature to match the actual behavior. the new
__syncall_wait function allows a __synccall callback to wait for other
threads to continue without returning, so that it can resume action
after the caller finishes. this interface could be made significantly
more general/powerful with minimal effort, but i'll wait to do that
until it's actually useful for something.
the new absolute-time-based wait kernelside was hard to get right and
basically just code duplication. it could only improve "performance"
when waiting, and even then, the improvement was just slight drop in
cpu usage during a wait.
actually, with vdso clock_gettime, the "old" way will be even faster
than the "new" way if the time has already expired, since it will not
invoke any syscalls. it can determine entirely in userspace that it
needs to return ETIMEDOUT.
normally we allow cancellation to be acted upon when a syscall fails
with EINTR, since there is no useful status to report to the caller in
this case, and the signal that caused the interruption was almost
surely the cancellation request, anyway.
however, unlike all other syscalls, close has actually performed its
resource-deallocation function whenever it returns, even when it
returned an error. if we allow cancellation at this point, the caller
has no way of informing the program that the file descriptor was
closed, and the program may later try to close the file descriptor
again, possibly closing a different, newly-opened file.
the workaround looks ugly (special-casing one syscall), but it's
actually the case that close is the one and only syscall (at least
among cancellation points) with this ugly property.
cleanup push and pop are also no-ops if pthread_exit is not reachable.
this can make a big difference for library code which needs to protect
itself against cancellation, but which is unlikely to actually be used
in programs with threads/cancellation.
previously, pthread_cleanup_push/pop were pulling in all of
pthread_create due to dependency on the __pthread_unwind_next
function. this was not needed, as cancellation cleanup handlers can
never be called unless pthread_exit or pthread_cancel is reachable.
like mutexes and semaphores, rwlocks suffered from a race condition
where the unlock operation could access the lock memory after another
thread successfully obtained the lock (and possibly destroyed or
unmapped the object). this has been fixed in the same way it was fixed
for other lock types.
in addition, the previous implementation favored writers over readers.
in the absence of other considerations, that is the best behavior for
rwlocks, and posix explicitly allows it. however posix also requires
read locks to be recursive. if writers are favored, any attempt to
obtain a read lock while a writer is waiting for the lock will fail,
causing "recursive" read locks to deadlock. this can be avoided by
keeping track of which threads already hold read locks, but doing so
requires unbounded memory usage, and there must be a fallback case
that favors readers in case memory allocation failed. and all of this
must be synchronized. the cost, complexity, and risk of errors in
getting it right is too great, so we simply favor readers.
tracking of the owner of write locks has been removed, as it was not
useful for anything. it could allow deadlock detection, but it's not
clear to me that returning EDEADLK (which a buggy program is likely to
ignore) is better than deadlocking; at least the latter behavior
prevents further data corruption. a correct program cannot invoke this
situation anyway.
the reader count and write lock state, as well as the "last minute"
waiter flag have all been combined into a single atomic lock. this
means all state transitions for the lock are atomic compare-and-swap
operations. this makes establishing correctness much easier and may
improve performance.
finally, some code duplication has been cleaned up. more is called
for, especially the standard __timedwait idiom repeated in all locks.
futex returns EINVAL, not ENOSYS, when op is not supported.
unfortunately this looks just like EINVAL from other causes, and we
end up running the fallback code and getting EINVAL again. fortunately
this case should be rare since correct code should not generate EINVAL
anyway.
new features:
- FUTEX_WAIT_BITSET op will be used for timed waits if available. this
saves a call to clock_gettime.
- error checking for the timespec struct is now inside __timedwait so
it doesn't need to be duplicated everywhere. cond_timedwait still
needs to duplicate it to avoid unlocking the mutex, though.
- pushing and popping the cancellation handler is delegated to
__timedwait, and cancellable/non-cancellable waits are unified.
this change is needed to fix a race condition and ensure that it's
possible to unlock and destroy or unmap the mutex as soon as
pthread_mutex_lock succeeds. POSIX explicitly gives such an example in
the rationale and requires an implementation to allow such usage.
the race condition these changes address is described in glibc bug
report number 12674:
http://sourceware.org/bugzilla/show_bug.cgi?id=12674
up until now, musl has shared the bug, and i had not been able to
figure out how to eliminate it. in short, the problem is that it's not
valid for sem_post to inspect the waiters count after incrementing the
semaphore value, because another thread may have already successfully
returned from sem_wait, (rightly) deemed itself the only remaining
user of the semaphore, and chosen to destroy and free it (or unmap the
shared memory it's stored in). POSIX is not explicit in blessing this
usage, but it gives a very explicit analogous example with mutexes
(which, in musl and glibc, also suffer from the same race condition
bug) in the rationale for pthread_mutex_destroy.
the new semaphore implementation augments the waiter count with a
redundant waiter indication in the semaphore value itself,
representing the presence of "last minute" waiters that may have
arrived after sem_post read the waiter count. this allows sem_post to
read the waiter count prior to incrementing the semaphore value,
rather than after incrementing it, so as to avoid accessing the
semaphore memory whatsoever after the increment takes place.
a similar, but much simpler, fix should be possible for mutexes and
other locking primitives whose usage rules are stricter than
semaphores.
previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.
i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.
this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.
changing credentials in a multi-threaded program is extremely
difficult on linux because it requires synchronizing the change
between all threads, which have their own thread-local credentials on
the kernel side. this is further complicated by the fact that changing
the real uid can fail due to exceeding RLIMIT_NPROC, making it
possible that the syscall will succeed in some threads but fail in
others.
the old __rsyscall approach being replaced was robust in that it would
report failure if any one thread failed, but in this case, the program
would be left in an inconsistent state where individual threads might
have different uid. (this was not as bad as glibc, which would
sometimes even fail to report the failure entirely!)
the new approach being committed refuses to change real user id when
it cannot temporarily set the rlimit to infinity. this is completely
POSIX conformant since POSIX does not require an implementation to
allow real-user-id changes for non-privileged processes whatsoever.
still, setting the real uid can fail due to memory allocation in the
kernel, but this can only happen if there is not already a cached
object for the target user. thus, we forcibly serialize the syscalls
attempts, and fail the entire operation on the first failure. this
*should* lead to an all-or-nothing success/failure result, but it's
still fragile and highly dependent on kernel developers not breaking
things worse than they're already broken.
ideally linux will eventually add a CLONE_USERCRED flag that would
give POSIX conformant credential changes without any hacks from
userspace, and all of this code would become redundant and could be
removed ~10 years down the line when everyone has abandoned the old
broken kernels. i'm not holding my breath...
this seems to be necessary to make the linker accept the functions in
a shared library (perhaps to generate PLT entries?)
strictly speaking libc-internal asm should not need it. i might clean
that up later.
if thread id was reused by the kernel between the time pthread_kill
read it from the userspace pthread_t object and the time of the tgkill
syscall, a signal could be sent to the wrong thread. the tgkill
syscall was supposed to prevent this race (versus the old tkill
syscall) but it can't; it can only help in the case where the tid is
reused in a different process, but not when the tid is reused in the
same process.
the only solution i can see is an extra lock to prevent threads from
exiting while another thread is trying to pthread_kill them. it should
be very very cheap in the non-contended case.
there is a resource limit of 0 bits to store the concurrency level
requested. thus any positive level exceeds a resource limit, resulting
in EAGAIN. :-)
the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.
note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.
the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.
these should be tweaked according to testing. offhand i know 1000 is
too low and 5000 is likely to be sufficiently high. consider trying to
add futexes to file locking, too...
the previous implementation had at least 2 problems:
1. the case where additional threads reached the barrier before the
first wave was finished leaving the barrier was untested and seemed
not to be working.
2. threads leaving the barrier continued to access memory within the
barrier object after other threads had successfully returned from
pthread_barrier_wait. this could lead to memory corruption or crashes
if the barrier object had automatic storage in one of the waiting
threads and went out of scope before all threads finished returning,
or if one thread unmapped the memory in which the barrier object
lived.
the new implementation avoids both problems by making the barrier
state essentially local to the first thread which enters the barrier
wait, and forces that thread to be the last to return.
some functions that should have been testing whether pthread_self()
had been called and initialized the thread pointer were instead
testing whether pthread_create() had been called and actually made the
program "threaded". while it's unlikely any mismatch would occur in
real-world problems, this could have introduced subtle bugs. now, we
store the address of the main thread's thread descriptor in the libc
structure and use its presence as a flag that the thread register is
initialized. note that after fork, the calling thread (not necessarily
the original main thread) is the new main thread.
we already checked before making the syscall, but it's possible that a
signal handler interrupted the blocking syscall and disabled
cancellation, and that this is the cause of EINTR. in this case, the
old behavior was testably wrong.
don't waste time (and significant code size due to function call
overhead!) setting errno when the result of a syscall does not matter
or when it can't fail.
x86_64 was just plain wrong in the cancel-flag-already-set path, and
crashing.
the more subtle error was not clearing the saved stack pointer before
returning to c code. this could result in the signal handler
misidentifying c code as the pre-syscall part of the asm, and acting
on cancellation at the wrong time, and thus resource leak race
conditions.
also, now __cancel (in the c code) is responsible for clearing the
saved sp in the already-cancelled branch. this means we have to use
call rather than jmp to ensure the stack pointer in the c will never
match what the asm saved.
the goal is to be able to use pthread_setcancelstate internally in
the implementation, whenever a function might want to use functions
which are cancellation points but avoid becoming a cancellation point
itself. i could have just used a separate internal function for
temporarily inhibiting cancellation, but the solution in this commit
is better because (1) it's one less implementation-specific detail in
functions that need to use it, and (2) application code can also get
the same benefit.
previously, pthread_setcancelstate dependend on pthread_self, which
would pull in unwanted thread setup overhead for non-threaded
programs. now, it temporarily stores the state in the global libc
struct if threads have not been initialized, and later moves it if
needed. this way we can instead use __pthread_self, which has no
dependencies and assumes that the thread register is already valid.
Rich Felker