diff options
Diffstat (limited to 'src/basic/process-util.c')
| -rw-r--r-- | src/basic/process-util.c | 66 |
1 files changed, 64 insertions, 2 deletions
diff --git a/src/basic/process-util.c b/src/basic/process-util.c index 6e8554626..4993de886 100644 --- a/src/basic/process-util.c +++ b/src/basic/process-util.c @@ -850,7 +850,7 @@ bool is_main_thread(void) { static thread_local int cached = 0; if (_unlikely_(cached == 0)) - cached = getpid() == gettid() ? 1 : -1; + cached = getpid_cached() == gettid() ? 1 : -1; return cached > 0; } @@ -915,7 +915,7 @@ const char* personality_to_string(unsigned long p) { void valgrind_summary_hack(void) { #ifdef HAVE_VALGRIND_VALGRIND_H - if (getpid() == 1 && RUNNING_ON_VALGRIND) { + if (getpid_cached() == 1 && RUNNING_ON_VALGRIND) { pid_t pid; pid = raw_clone(SIGCHLD); if (pid < 0) @@ -959,6 +959,68 @@ int ioprio_parse_priority(const char *s, int *ret) { return 0; } +/* The cached PID, possible values: + * + * == UNSET [0] → cache not initialized yet + * == BUSY [-1] → some thread is initializing it at the moment + * any other → the cached PID + */ + +#define CACHED_PID_UNSET ((pid_t) 0) +#define CACHED_PID_BUSY ((pid_t) -1) + +static pid_t cached_pid = CACHED_PID_UNSET; + +static void reset_cached_pid(void) { + /* Invoked in the child after a fork(), i.e. at the first moment the PID changed */ + cached_pid = CACHED_PID_UNSET; +} + +/* We use glibc __register_atfork() + __dso_handle directly here, as they are not included in the glibc + * headers. __register_atfork() is mostly equivalent to pthread_atfork(), but doesn't require us to link against + * libpthread, as it is part of glibc anyway. */ +extern int __register_atfork(void (*prepare) (void), void (*parent) (void), void (*child) (void), void * __dso_handle); +extern void* __dso_handle __attribute__ ((__weak__)); + +pid_t getpid_cached(void) { + pid_t current_value; + + /* getpid_cached() is much like getpid(), but caches the value in local memory, to avoid having to invoke a + * system call each time. This restores glibc behaviour from before 2.24, when getpid() was unconditionally + * cached. Starting with 2.24 getpid() started to become prohibitively expensive when used for detecting when + * objects were used across fork()s. With this caching the old behaviour is somewhat restored. + * + * https://bugzilla.redhat.com/show_bug.cgi?id=1443976 + * https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1d2bc2eae969543b89850e35e532f3144122d80a + */ + + current_value = __sync_val_compare_and_swap(&cached_pid, CACHED_PID_UNSET, CACHED_PID_BUSY); + + switch (current_value) { + + case CACHED_PID_UNSET: { /* Not initialized yet, then do so now */ + pid_t new_pid; + + new_pid = getpid(); + + if (__register_atfork(NULL, NULL, reset_cached_pid, __dso_handle) != 0) { + /* OOM? Let's try again later */ + cached_pid = CACHED_PID_UNSET; + return new_pid; + } + + cached_pid = new_pid; + return new_pid; + } + + case CACHED_PID_BUSY: /* Somebody else is currently initializing */ + return getpid(); + + default: /* Properly initialized */ + return current_value; + } +} + static const char *const ioprio_class_table[] = { [IOPRIO_CLASS_NONE] = "none", [IOPRIO_CLASS_RT] = "realtime", |