Calling system() in multi-threads.

Fri Jul 18 08:42:47 GMT 2025

On 2025年7月18日 09:54:20 +0200
Corinna Vinschen wrote:
> On Jul 18 01:28, Takashi Yano via Cygwin wrote:
> > On 2025年7月18日 00:44:46 +0900
> > Takashi Yano wrote:
> > > On 2025年7月17日 17:19:49 +0200
> > > Corinna Vinschen wrote:
> > > > On Jul 17 23:14, Takashi Yano via Cygwin wrote:
> > > > > Hi Corinna,
> > > > > 
> > > > > On 2025年7月16日 17:36:42 +0200
> > > > > Corinna Vinschen wrote:
> > > > > > On Jul 16 23:52, Takashi Yano via Cygwin wrote:
> > > > > > > Do you have any idea?
> > > > > > 
> > > > > > Locking would be super-simple.
> > > > > > 
> > > > > > But theoretically it should be possible to use a local child_info_spawn
> > > > > > variable at this point. The ch_spawn child_info_spawn instance is not
> > > > > > copied to the child anyway, so that should be safe. The same goes for
> > > > > > posix_spawn() then, btw.
> > > > > > 
> > > > > > I checked the sources and I don't see any dependency to ch_spawn
> > > > > > from a spawning process, in contrast to an exec'ing process. That
> > > > > > doesn't mean there is none, just that I didn't find any.
> > > > > 
> > > > > Thanks!
> > > > > As a starting point, I tried tntroducing locking. It almost works
> > > > > as expected, however, sometimes my STC in my first report is hangs
> > > > > if N is large e.g. 100. The patch is as attached.
> > > > > 
> > > > > What am I missing?
> > > > 
> > > > I don't know. You're perhaps not releasing the lock in all cases.
> > > > But I would have to debug this just like you ¯\_(ツ)_/¯
> > > > 
> > > > Out of curiosity, did you try using a locale child_info_spawn instance
> > > > instead? That would be a rather nice solution, but I'm pretty sure
> > > > there's some other problem lurking in the dark...
> > > 
> > > I'm not sure what to do with local child_info_spawn.
> > > Some of other modules refer to ch_spawn, such as exception.cc and
> > > pinfo.cc. Also, has_execed* uses ch_spawn. What should we do for that?
> > > 
> > > I've just tried simply the following patch, however, this also hangs
> > > with my STC.
> > > 
> > > diff --git a/winsup/cygwin/spawn.cc b/winsup/cygwin/spawn.cc
> > > index cb58b6eed..56fca6e45 100644
> > > --- a/winsup/cygwin/spawn.cc
> > > +++ b/winsup/cygwin/spawn.cc
> > > @@ -944,6 +944,7 @@ spawnve (int mode, const char *path, const char *const *argv,
> > > int ret;
> > > 
> > > syscall_printf ("spawnve (%s, %s, %p)", path, argv[0], envp);
> > > + child_info_spawn ch_spawn_local;
> > > 
> > > if (!envp)
> > > envp = empty_env;
> > > @@ -951,7 +952,7 @@ spawnve (int mode, const char *path, const char *const *argv,
> > > switch (_P_MODE (mode))
> > > {
> > > case _P_OVERLAY:
> > > - ch_spawn.worker (path, argv, envp, mode);
> > > + ch_spawn_local.worker (path, argv, envp, mode);
> > > /* Errno should be set by worker. */
> > > ret = -1;
> > > break;
> > > @@ -961,7 +962,7 @@ spawnve (int mode, const char *path, const char *const *argv,
> > > case _P_WAIT:
> > > case _P_DETACH:
> > > case _P_SYSTEM:
> > > - ret = ch_spawn.worker (path, argv, envp, mode);
> > > + ret = ch_spawn_local.worker (path, argv, envp, mode);
> > > break;
> > > default:
> > > set_errno (EINVAL);
> > 
> > The hang seems to be at acquiring the cygheap_protect lock in child sh.exe.
> > This lock is aquired only in _cfree() and _cmalloc(), so I am not sure why
> > cygheap_protect cannot be acquired at this point at all...
>> How do the affected backtraces look like?
>> Also, one reason could be that cygheap_protect is a SRWLOCK since
> 5d3e79ec6bb73 ("Cygwin: cygheap: use SRWLOCK for cygheap locking")
>> SRWLOCK is not recursive. What if you revert this lock to a muto as
> before 5d3e79ec6bb73?

Thanks! I'll try muto.
-- 
Takashi Yano <takashi.yano@nifty.ne.jp>