getpid.2: Note that PID caching is removed as of glibc 2.25

Since glibc 2.25 the PID cache is removed.

Rationale given in the release notes:
https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal
~~~
3.2.3. Calls to getpid are no longer cached

The PID cache used by glibc has been removed. In certain scenarios
the cache was not 100% reliable and because of that it was deemed
safer to remove the cache than to potentially return a wrong
answer.

Applications performing getpid() calls in a loop will see the
worst case performance degradation as the library call will
perform a system call at each invocation. Such application uses
were known to exist at least in OpenSSL (fork()-based PRNG
invalidation), but supporting the performance of that specific
invalidation mechanism was not judged to have sufficient value
against immediate and long-term benefits of removing the cache.

Functional reasons exist for the PID cache removal including
problems with PID namespaces, interoperability with raw system
calls (BZ#17214, Chrome: Issue 800183004), and improvements to
spawn (BZ#19957). Performance is actually increased in
pthread_create() with the removal of the cache since the
implementation no longer needs to perform an invalidation step.

Applications performing getpid() in a loop that need to do some
level of fork()-based invalidation can instead use
pthread_atfork() to register handlers to handle the invalidation.
There is work-in-progress to make pthread_atfork() available to
applications that do not link against libpthread.so (Provide
pthread_atfork() without libpthread.so).

Other kinds of invalidation are not supported and the glibc
community will actively look at a kernel assisted mechanism for
state management across fork(), vfork(), clone() and other
interfaces which can benefit from such semantics. It is the same
type of solution required for crypto PRNG reset across such API
calls.
~~~

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/getpid.2

@@ -112,6 +112,11 @@ for discussion of a case where
 may return the wrong value even when invoking
 .BR clone (2)
 via the glibc wrapper function.
+
+Since glibc version 2.25, the PID cache is removed, and calls to
+.BR getpid ()
+will not be cached.
+
 .SH SEE ALSO
 .BR clone (2),
 .BR fork (2),