Consider the following program (test.cc):

    #include <pthread.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <errno.h>
    
    #define C(c) if(int e = (c)) { errno = e; perror(#c); exit(EXIT_FAILURE); }
    
    int main() {
        pthread_mutexattr_t attr;
        pthread_mutex_t plock;
        C(pthread_mutexattr_init(&attr));
        C(pthread_mutexattr_setprioceiling(&attr, 99));
        C(pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_PROTECT));
        C(pthread_mutex_init(&plock, &attr));
        C(pthread_mutexattr_destroy(&attr));
    
        if(int e = pthread_mutex_lock(&plock)) {
            errno = e;
            perror("1st pthread_mutex_lock");
            if(int e = pthread_mutex_lock(&plock)) {
                errno = e;
                perror("2nd pthread_mutex_lock");
            }
            else {
                printf("2nd pthread_mutex_lock succeeded\n");
            }
        }
        else {
            printf("1st pthread_mutex_lock succeeded\n");
        }
    }

Compiled as:

    $ g++ --version
    g++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
    Copyright (C) 2018 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
    
    $ g++ -o test -O2 -W{all,extra,error} -pthread test.cc
    
    $ ldd test
        linux-vdso.so.1 (0x00007fffa69b2000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa4e445b000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa4e406a000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fa4e487d000)
    
    $ /lib/x86_64-linux-gnu/libc.so.6
    GNU C Library (Ubuntu GLIBC 2.27-3ubuntu1) stable release version 2.27.
    Copyright (C) 2018 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.
    There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
    PARTICULAR PURPOSE.
    Compiled by GNU CC version 7.3.0.
    libc ABIs: UNIQUE IFUNC
    For bug reporting instructions, please see:
    <https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.

Now let's run it:

    $ ./test
    1st pthread_mutex_lock: Invalid argument
    2nd pthread_mutex_lock succeeded
    
    $ sudo chrt --fifo 1 ./test
    1st pthread_mutex_lock succeeded

The question is: why does the 1st pthread_mutex_lock fails but the 2nd pthread_mutex_lock succeeds when the calling thread has wrong priority class for the priority ceiling?