Hi,

Attached is the patch for the previously mentioned issues of 
clnt_create() deadlocking on errors, and cond_signal() being unprotected 
against asyncronous calls to clnt_destroy().

cheers,

-- A.


On 7/20/22 14:16, Attila Kovacs wrote:

More mutex issues -- this time more likely to do with the connect 
deadlock we see:

in clnt_dg.c, in clnt_dg_create():


clnt_fd_lock is locked on L171, but then not released if jumping to the 
err1 label on an error (L175 and L180). This means that those errors 
will deadlock all further operations that require clnt_fd_lock access.

Same in clnt_vc.c in clnt_vc_create, on lines 215, 222, and 230 
respectively.

-- A.





On 7/20/22 12:09, Attila Kovacs wrote:

quoted

Hi Steve,


We are using the tirpc library in an MT environment. We are 
experiencing occasional deadlocks when calling clnt_create_timed() 
concurrently from multiple threads. When this happens, all connect 
threads hang, with each thread taking up 100% CPU.

I was peeking at the code (release 1.3.2), and some potential problems 
I see is how waiting / signaling is implemented on cu->cu_fd_lock.cv 
in clnt_dg.c and clnt_vc.c.

1. In cnlt_dg_freeres() and clnt_vc_freeres(), cond_signal() is called 
after unlocking the mutex (clnt_fd_lock). The manual of 
pthread_cond_signal() allows that, but mentions that for consistent 
scheduling, cond_signal() should be called with the waiting mutex 
locked.  (i.e. lock -> signal -> unlock, rather than lock -> unlock -> 
signal).

One of the dangers of signaling with the unlocked mutex, is that in 
MT, another thread can lock the mutex before the signal call is made, 
and potentially destroy the condition variable (e.g. an asynchronous 
call to clnt_*_destroy()). Thus, by the time the signal() is called in 
clnt*_freeres(), both the condition may be invalid.

The proper sequence here should be:

     [...]
     mutex_lock(&clnt_fd_lock);
     while (ct->ct_fd_lock->active)
         cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
     xdrs->x_op = XDR_FREE;
     dummy = (*xdr_res)(xdrs, res_ptr);
     thr_sigsetmask(SIG_SETMASK, &(mask), NULL);
     cond_signal(&ct->ct_fd_lock->cv);
     mutex_unlock(&clnt_fd_lock);


2. Similar issue in the macro release_fd_lock() in both the vc and dg 
sources. Here again the sequence ought to be:

#define release_fd_lock(fd_lock, mask) {    \
     mutex_lock(&clnt_fd_lock);    \
     fd_lock->active = FALSE;    \
     thr_sigsetmask(SIG_SETMASK, &(mask), (sigset_t *) NULL); \
     cond_signal(&fd_lock->cv);    \
     mutex_unlock(&clnt_fd_lock);    \
}


3. The use of cond_wait() in clnt_dg.c and clnt_vc.c is also 
unprotected against the asynchronous destruction of the condition 
variable, since cond_wait() releases the mutex as it enters the 
waiting state. So, when it is called again in the while() loop, the 
condtion might no longer exist. Thus, before calling cond_wait(), one 
should check that the client is valid (not destroyed) inside the wait 
loop.


I'm not sure if these particular issues are the cause of the deadlocks 
we see, but I think they are problematic enough on their own, and 
perhaps should be fixed in the next release.

Thanks,

-- Attila