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# Process Model Unification

The Solaris Process Model Unification project:

PSARC/2002/117 Solaris Process Model Unification

4470917 Solaris Process Model Unification

folded libthread into libc and has led to some fundamental changes

in the rules by which code in libc must be developed and maintained.

All code in libc must be both MT-Safe and Fork-Safe

and where possible (almost everywhere), Async-Signal-Safe.

To this end, the following rules should be followed:

Almost all internal libc locks (mutexes and read-write locks)

should be acquired and released via these interfaces:

mutex_t some_lock = DEFAULTMUTEX;

lmutex_lock(&some_lock);

... do something critical ...

lmutex_unlock(&some_lock);

rwlock_t some_rw_lock = DEFAULTRWLOCK;

lrw_rdlock(&some_rw_lock);

... multiple threads can do something ...

lrw_unlock(&some_rw_lock);

lrw_wrlock(&some_rw_lock);

... only one thread can do something ...

lrw_unlock(&some_rw_lock);

The above l* versions of the mutex and rwlock interfaces do more

than the ordinary interfaces: They define critical regions in

which the calling thread cannot be suspended (making the region

fork-safe) and in which the calling thread has all signals deferred

(making the region async-signal-safe).

However, certain rules apply to the code within these critical regions:

- The code must be of guaranteed short duration; no calls

to interfaces that might block indefinitely are allowed.

This means no calls into stdio or syslog() and no calls

to cond_wait() unless there is a guarantee of an almost-

immediate call to cond_signal() or cond_broadcast()

from elsewhere.

- The code cannot call any non-l* synchronization

primitives (mutex_lock(), _private_mutex_lock(),

rw_wrlock(), rw_rdlock(), sema_wait(), etc.)

- The code cannot call any functions outside of libc,

including application callbacks and functions from

dlopen()ed objects, such as those in the I18N code.

- Because malloc(), calloc(), realloc(), and free()

are designed to be interposed upon, they fall into

the previous case of prohibition. None of these can

be called by a thread while in a critical region.

There is a private memory allocator for use internally to libc.

It cannot be interposed upon and it is safe to use while in

a critical region (or for that matter while not in a critical

region; it is async-signal-safe and fork-safe):

void *lmalloc(size_t);

void lfree(void *, size_t);

void *libc_malloc(size_t);

void *libc_realloc(void *, size_t);

char *libc_strdup(const char *);

void libc_free(void *);

lmalloc() and lfree() are the basic interfaces. The libc_*()

variants are built on top of lmalloc()/lfree() but they have

the same interface signatures as the corresponding functions

without the 'libc_' prefix. lmalloc() and libc_malloc()

return zeroed memory blocks. Note that lmalloc()/lfree()

require the caller to remember the size parameter passed

to lmalloc() and to pass the same value to lfree().

Memory allocated by lmalloc() can only be freed by lfree().

Memory allocated by libc_malloc(), libc_realloc(), or libc_strdup()

can only be freed by libc_free(). Never pass such allocated

memory out of libc if the caller of libc is expected to free it.

lmalloc()/lfree() is a small and simple power of two allocator.

Do not use it as a general-purpose allocator. Be kind to it.

There is a special mutual exclusion interface that exists for

cases, like code in the I18N interfaces, where mutual exclusion

is required but the above rules cannot be followed:

int fork_lock_enter(const char *);

void fork_lock_exit(void);

fork_lock_enter() does triple-duty. Not only does it serialize

calls to fork() and forkall(), but it also serializes calls to

thr_suspend() (fork() and forkall() also suspend other threads),

and furthermore it serializes I18N calls to functions in other

dlopen()ed L10N objects that might be calling malloc()/free().

Use it in general like this:

(void) fork_lock_enter(NULL);

... serialized; do something that might call malloc ...

fork_lock_exit();

The 'const char *' argument to fork_lock_enter() should always

be NULL except for two special cases:

- When called from fork() or forkall()

- When called from pthread_atfork()

This enforces the prohibition against calling fork() or pthread_atfork()

from a pthread_atfork()-registered fork handler function while a fork()

prologue or epilogue is in progress. If _THREAD_ERROR_DETECTION is set

to 1 or 2 in the environment, such cases will draw a nasty message and

will dump core if _THREAD_ERROR_DETECTION=2. fork_lock_enter() returns

non-zero only if it is called from a fork handler. This is of interest

only to callers that have to do something about this condition; the

return value should be ignored in all other cases (fork_lock_enter()

never actually fails).

It is an error to call fork_lock_enter() while in a critical region

(that is, while holding any internal libc lock).

On return from fork_lock_enter(), no internal libc locks are held

but a flag has been set to cause other callers of fork_lock_enter()

to delay (via _cond_wait()) until fork_lock_exit() is called.

These are the rules to follow for memory allocation:

- If a function acquires an internal libc lock or is called while

an internal libc lock is held:

* The malloc family cannot be used.

* lmalloc or libc_malloc should be used. The memory must

be released by lfree or libc_free, respectively.

* lfree takes an argument to tell the size of the releasing

memory. If the function does not know the size at the

releasing point, libc_malloc and libc_free should be used.

* As the memory allocated by lmalloc or libc_malloc needs

to be released by lfree or libc_free and these are internal

to libc, they cannot be used to allocate memory that might

be released by application code outside libc.

* If the memory allocation by malloc() cannot be avoided and

the scalability of the function does not matter much, the

function can be serialized with fork_lock_enter() instead

of lmutex_lock().

* If the memory allocation by malloc() cannot be avoided and

the scalability of the function does matter, another

implementation of the function will be necessary.

In a DEBUG build of libc:

make THREAD_DEBUG=-DTHREAD_DEBUG install

many of these rules are enforced by ASSERT() statements scattered about

in the libc sources. This is the default mode for building libc when

a DEBUG nightly build is performed.

# libaio/librt Implementation In libc

The putback of the project:

6416832 libaio and librt can and should be folded into libc

introduced several libc-private locking interfaces:

void sig_mutex_lock(mutex_t *);

void sig_mutex_unlock(mutex_t *);

int sig_mutex_trylock(mutex_t *);

int sig_cond_wait(cond_t *, mutex_t *);

int sig_cond_reltimedwait(cond_t *, mutex_t *, const timespec_t *);

which are declared in both "thr_uberdata.h" and "mtlib.h".

They are used in specialized code in libc, like the asynchronous i/o code.

Unlike the lmutex_lock() and lmutex_unlock() interfaces described above,

these interfaces do not define critical regions, but signals are

deferred while locks acquired by these functions are held, making

their use be async-signal safe. Calls to malloc(), calloc(), realloc(),

and free() are permissible while holding such locks.

These interfaces were brought over from code in the former libaio

and librt and are necessary because, where they are used, the code

must execute potentially long-term waits and must be cancelable.

sig_cond_wait() and sig_cond_reltimedwait() are cancellation points.

These interfaces are available for other uses inside libc, as

the need arises. (There is no need if the code does not perform

long-term waits.) Just follow a few rules to be self-consistent:

- Don't mix calls to mutex_[un]lock(), lmutex_[un]lock() and

sig_mutex_[un]lock() on the same mutex.

- Don't call cond_wait() with a mutex acquired by sig_mutex_lock();

call sig_cond_wait() or sig_cond_reltimedwait().

- Use pthread_cleanup_push() and pthread_cleanup_pop() to make

your code cancellation-safe.

- The sig_*() interfaces are not in themselves fork-safe.

You have to employ other logic to make your code fork-safe.

See the tail of postfork1_child() for examples.

# Removal Of Synonym Symbols

The following project:

PSARC 2008/309 expunge synonyms.h

6700179 expunge synonyms.h

Removed the historical "synonym" symbols from the C Library.

Historically, for every public function symbol in the C library a second,

private, symbol of the same value was defined to be used internally by libc

(generally, one or the other will be a weak symbol, precisely which is

inconsistent).

These synonym symbols existed such that an application which provided

otherwise conflicting symbols could interpose on the version in libc without

compromising libc itself, that is if libc defines fopen() which needs open() it

would call _open() and an application defined open() would not cause fopen()

to break. This was made transparent to code within libc via a header,

synonyms.h, which would #define open _open, for all such symbols.

Since ON now uses direct bindings extensively all symbols not explicitly

marked "NODIRECT" are directly bound within libc anyway, and none of this is

actually necessary. Thus synonyms.h was removed, and no new synonym symbols

need be added. However, unfortunately, certain of the private symbols were

inadvertently exposed to applications, and several are known to have been

used, thus these existing synonyms must continue to exist to maintain

compatibility. A preloadable library, /lib/c_synonyms.so.1 is provided which

also provides the historical names with underscore prefixes to allow any other

incorrect application to continue to function.

It should never be necessary to add additional synonym symbols to libc nor to

add underscore prefixed aliases to c_synonyms.so.1.

# libc Internals Scoped Protected

The integration of the fix for:

6689238 libc needs global protection against ld.so.1

Scopes all function symbols within libc protected, excepting those that need

to be accepting of interposition and to have a consistent version called both

within and without libc (basically, the malloc() family).

This causes references by libc to itself to be permanently and unavoidably

bound, and thus to never enter ld.so (and potentially from there audit

libraries or other such support libraries). This maintains an otherwise

complicated to verify invariant: within critical sections (with any internal

lock held, etc) execution can never leave the context of libc. Previously

this was done with a selection of known-to-be-problematic functions having

weak synonyms scoped private, but this was both difficult to verify, difficult

to remember, and thus always at least somewhat incomplete.

In summary, any new function symbol in libc must be scoped protected unless it

is one of a very small group of functions that must allow interposed versions

to be bound to from the C library itself -- it is grossly unlikely that more

of these will occur.